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Some sites comes with redirectors or "beacons" for detecting user presence. Example i'm on site X page A and there i click on link to go on page B. But due marketing department this pass via HTTP redirector or pure HTTP (and there 301 redirect to HTTPS). Then this page B can be not indexed.

This mean that once you set sitewide 301 redirect to encrypted connection you must make few more steps:

• you must check all resources to pass via this encrypted channel. Images, CSS, JS - just anything.
• you must check canonical to be set to HTTPS
• you must check that link between pages to be also HTTPS
• you must see any 3rd party tools for encrypted connection. Can be analytics software or "tracking pixels" or heat maps or ads.
• you must check if outgoing links from your site can be via other sites with encryption. Can be Wikipedia, Moz, Google. Since everything there is already encrypted you will skip frustrating HTTPS -> HTTP -> HTTPS jump too.

So then your site can be indexed in HTTPS. It's tricky procedure with many traps.

So you need to get all links from:

• SearchConsole
• Ahrefs
• Moz OSE
• Majestic - you need to verify your domain and then you can download fresh list for free. For historical you need to paying account.
• WebMeUp - free with some limitations. Paid version also available

Once you get everything you need to compose mega sheet in Excel (or Google Sheets) and then you can do "link profile audit".

But you also need to make "panda audit". And this is little bit weird because you need to read almost everything from many authority persons. From all of them i can recommend Glenn Gabe and Josh Bachynski. First can be found on http://www.hmtweb.com and second http://themoralconcept.net/pandalist.html Of course they also wrote for other sites like Moz, SEL, SEJ, etc. Of course this doesn't mean that there isn't other authors like:
https://moz.com/blog/have-we-been-wrong-about-panda-all-along

So let's go back on Panguin tool. You need very depth inspection in Analytics on your dates and dates of releasing algos.:
https://moz.com/google-algorithm-change
and make detail analysis of your situation. IMHO there is also huge chance that you're hit with both algos.

You can also have multiple sitemaps on 3rd sites. Look at Moz robots.txt:
Sitemap: https://moz.com/blog-sitemap.xml
Sitemap: https://moz.com/ugc-sitemap.xml
Sitemap: https://moz.com/profiles-sitemap.xml
Sitemap: http://d2eeipcrcdle6.cloudfront.net/past-videos.xml
Sitemap: http://app.wistia.com/sitemaps/36357.xml

Also Google.com robots.txt:
Sitemap: http://www.gstatic.com/culturalinstitute/sitemaps/www_google_com_culturalinstitute/sitemap-index.xml
Sitemap: http://www.gstatic.com/dictionary/static/sitemaps/sitemap_index.xml
Sitemap: http://www.gstatic.com/earth/gallery/sitemaps/sitemap.xml
Sitemap: http://www.gstatic.com/s2/sitemaps/profiles-sitemap.xml
Sitemap: http://www.gstatic.com/trends/websites/sitemaps/sitemapindex.xml
Sitemap: https://www.google.com/sitemap.xml

Also Bing.com robots.txt:
Sitemap: http://cn.bing.com/dict/sitemap-index.xml
Sitemap: http://www.bing.com/offers/sitemap.xml

So using multiple sitemaps it's OK and they can be also hosted on 3rd party server.

Or you can leave but change their links to pass some URL shortener - bit.ly or t.co until they comes with HTTPS version.

Or you can also make some page as "partners" where you can link only HTTP external sites.

Or you can also make internal page redirector to HTTP site. Like HTTPS -> HTTPS (inside redirector and dummy page) -> HTTP. On this case redirector won't be indexed and that's why it's dummy.

And this is just three ideas that i think for one minute. Probably mine favorite is #3. But it's IMHO.

TL;DR -your DA is small because your business is small. Period.

Domain authority is just a number. PR is also number.

You can rank very well also with little DA, same is with PR. Focusing with DA improvements is meaningless because DA can become higher with links/linkbuilding. And links can be earned with good content or forced with SEO linkbuilders. This definitely can improve little bit your DA. Even in SEO there is term "PR sculpting".

Playing with DA and site is same as "which came first, the chicken or the egg?". If your site grows with links then DA will be increased. But growing from 40 to 50 is same as growing from 0 to 40 as links. And from 50 -> 60 is same as 0 to 50. All happening due their logarithmic scale for parameters. And this require huge works.

So let's go back on "chicken/egg" problem - you can improve DA if you get links from trusted newspapers with sites as Guardian, Independent, Mirror, Daily Express, Telegraph, etc. Also can be trusted magazines with sites - Glamour, Elle, Wired, etc. You can get links from TVs - BBC, ITV, C4, C5, etc. You can get links sponsoring events - like BigBrother, X-Factor, etc. Then your DA will become higher. Now let's face question again - you're small and to make that media to talk about you need to grow to national wide and become giant in your niche.

And how you grow is different story that we can't answer without market research, financial analysis and some forecasts.

Well if both sitemaps are for same site then it's OK. But it's much better to implement hreflang as this is explained here:https://support.google.com/webmasters/answer/2620865?hl=en

I'm not sure that Magento can do this but you always can hire 3rd party dev for building plugin/module for this.

Just as Russ says - many things may happen. I will add few more:

• Panda 4.2 https://moz.com/google-algorithm-change#2015 even was on 17 July in UK SERP can be delayed to August. You also can be hit from Panda at any time. Just recovering is slow.
• Penguin can hit you any time. You need to check and keep one eye on your backlinks. Just recovering is slow.
• You can be hit from hosting infrastructure, CMS theme update, CMS plugin update, etc.
• You should investigated previous searches in SearchConsole and compare this to actual one. You can use archive.org and see historical changes in site and it's pages.
• You should check website log. Similar to "ship captain's log book" you and team there should log every change to site and it's pages/content. And even small change can invoke disaster after 2-3 months or even longer.

I wrote few things that need to be seen. I can wrote even longer but sometime there are even outside of SEO events. Imagine that you're auditing something as "Rover cars" and there is sudden drop... because company is now defunct. You also can be hit from negative campaign, rumors, etc. As you can see it isn't hard to explain what's happening without know this customer, it's niche, competitors and social networks.

Have you change URL structure? Because new pages get PA of 1. If old pages are redirected with 301 to new pages then PA will be recovered on next Moz index update. If wasn't then do it ASAP.

You need to see your GA for traffic drops and web acces log files for 404 errors.

About H1, if page doesn't have H1 then will be used H2 or H3. Rule is "highers header available". But H1 is much better.

Redirects:
yyxysh.com -> DA=8; UPA = 17 in OSE, API {"pda":7.946532082305239,"upa":16.535296818533226}
www.yyxysh.com -> DA=8; UPA=21; {"pda":7.946532082305239,"upa":21.185637908436604}

zaegi.com -> 17/27; {"pda":17.31598169835115,"upa":27.430491280580874}
www.zaegi.com -> 17/13; {"pda":17.31598169835115,"upa":13.339025908887258}

As you can see API and OSE works. But checking different domain screw test. Please check your software did add "www" in beginning of checks? Because mine tests shows that it ADD it.

If you have Mac then you can use mine SEOAuditor very small app that return few important metrics:
http://www.mobiliodevelopment.com/seoauditor/
Actual API examples are from it. I just enable "debug log" to get raw results.

Yes - in Unicode there are too many quotes and seems that pasting different quote make test OK for humans but garbage for bots and browsers.

TL;DR - word count of WHAT?

Different tools have different metrics. Let's bust myth of HTML wordcounts!

So first open both pages and view their sources. Key have 9575 lines and Wickes is just 1854. Seems that one of page is little bit large. Let's try this with tool:
http://www.seoreviewtools.com/bulk-web-page-word-count-checker/
just paste both links, solve captcha and push button. And you will see that Key is with more keywords. Now let's try with tool #2:
http://textmechanic.com/text-tools/basic-text-tools/count-characters-words-lines/
just go in Key (not in page source, just view page in browser), select all, copy, go in tool #2 and paste. Make same check with Wickes.

Why this happens? Because different methodology of calculation. First tool go in HTML and grab everything in as words for calculation with stripped HTML tags, special symbols, etc. He show us number X and we are agreed that this IS correct number. In second tool you paste (BTW - you can make same calculation with Word, just go in Paste Special and paste as TEXT ONLY!) everything what user see and he show you correct number of words like Y.

So due different methodology why X is different than Y? Because you may have some ajax code, invisible layouts, and/or other HTML mess. For example in Key site i can clearly see DIV with display:none with heavy content. And this happens 61 times!

Now i hope that this give you answer why X and Y are different and both are correct at same time. Because counting methodology is different.

Real case... almost 10 years ago SEO manager in some company was flying to their HQ just to explain why their PR was dropped from 6 to 5. C-level execs wasn't happy about that.

Now fun part - impressions, CTR and visits was increased during that update.

And in addition to Trevor - get some links. I couldn't find links to your site in OSE. Links is still essential part of algorithms for ranking.

Also check ranking in SearchConsole in SearchAnalytics. Do you get also impressions there?

And last one - your site is relative new. Dec 2015? You are maybe still in sandbox and this prevent you in rankings.

I just browse via Google.IT for "vangogh" and you're on N:2 spot after Italian Wikipedia article. I strongly recommend you to create account in SearchConsole and verify your site. There you can see in SearchTraffic exactly what position you get in SERP results.

Now answer for browsers - modern search engines return SERP pages optimized for specific browsers. That's why it's possible browser X to see some results, browser Y to get other results. There is no way to escape this unless you change "user-agent" string in browser with some plugin. Some engines are even going further that return personalized results based on IP, previous searches from this IP, previous searches from this account, previous clicks on SERP, etc.

The only way to get right position is with SearchConsole or special tracking tools as Moz RankTracker. And there you can see "average position".

No,

Corporate contacts and corporate logo are part of Schema.org/Organization.

Example:

is valid and:

is also valid. Also valid is this:

All these JSON-LD can be validated w/o problems.

You can't add two meta descriptions and there isn't workaround. Seems as CMS limitation.

BUT! There is something much more scary - you have lot of duplicate content because you have two versions of each page. You can fix it with adding on mobile version canonical to the desktop version. And you must put rel="alternate" media="only screen and (max-width: 640px)" on desktop with link to mobile version. Everything is described very clear here:
https://developers.google.com/webmasters/mobile-sites/mobile-seo/separate-urls

The case is when you have special website that serve different versions of site based on user agent. So in this case you want to see what GoogleBot could retrieve from your site. This also can assist you in mobile redirects too also based on user agents.

In only one way where can't help you - it's responsive web because there bots see only one version of HTML.

Not only metadata is in French - whole homepage is in French!:
http://webcache.googleusercontent.com/search?q=cache:Acm7RY_EpzEJ:www.revolveclothing.com/&hl=en&gl=bg&strip=1&vwsrc=0

Seems that bot index this content in French. You need to go in SearchConsole and reindex this page and put to index.

True - potential impact can't be seen. But we have few awesome articles about this:
http://www.stateofdigital.com/optimising-urls-seo-ux/
https://moz.com/blog/site-architecture-for-seo
https://moz.com/blog/ugly-seo-mess-recovery-case-study <- checkout "flat structure"

So you now don't have silo structure in your site and he didn't works works. There are many authors about silo structure - Yoast, Bruce Clay, Glenn Gabe, etc. I can suggest to educate little bit about that and implement some structure. Of course redirect 301 old pages to new ones.

I think that A1 Sitemap Generator support this function.