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So you are asking on Moz if you should steal content from Moz then not credit them to increase your search ranking, and then your concern is not if what you are doing is wrong (it is) but only if Google will penalize you for it?  I believe that if you want content for your site you should make it or pay for it, not steal it, if you write optimized content then maybe people would expect copy on your site to be written by you.

Write as much copy as you need to convey the information that is needed for your visitor on that page.  If you can create a good user experience on one page with 100 words (by answering the pain that they would be going to that page to solve) and then you need 1,000 words for your next page, then do that to.  Don't make arbitrary copy limits or goals based on SEO, make your pages user friendly and your copy useful and your pages will rank well for the appropriate content.

While they as a whole have high page authority, when you create a sub domain blog on them, it does not get the same.  You should worry about creating good content on your site instead of making free blogs to put content onto to then push to your site.  Your best bet would be to set up a blog onto your website so that you can have new, relevant content being consistently added to your site.

You are moving the wrong way, Google has said that they prefer sites to be secure and that this will be a factor more and more.  As per your question, yes if you create 301 redirects your SEO losses will be minimal (and don't forget to update your GWMT).

You should disavow those links, if they are not effecting you now, they will eventually.

As for why you are not back to the top, you changed your site's focus so you lost all of the historic "mojo" that you had for those keywords.  Even now that you have put them back, what was lost is gone, you will need to slowly, overtime rebuild to get get back to what you had before.

If you cannot dedicate the time to SEO, bringing in a professional may be your best option.  They can help you get on the path to recovery and work with you to hopefully improve past where you started while fixing what this other company did.

Yes all of those can negatively effect your ranking.

I would guess it has to do with a lack of any actual content on the main page, and all of the other pages "content" being aggregated from other sites where it was originally posted.  A site like this would need either a large amount of advertising to drive traffic or in my opinion a complete overhaul and reevaluation of the content strategy.

A missing title is your meta title, it is the words you see in the tabs on your browser.

H1 elements are in your copy, each page should have one, and it should be the main focus of the page.

While you should not fear change, your comment "However, the current setup is sending a worthwhile traffic volume to the site." says all you need to know, if the service + city landing page model is working, don't change it to change it or because you read something that says you should.  If you do however believe that targeting the broad keywords instead will net you better local results, set up a few landing pages for the broad keywords and A/B test them against the local landing pages.

That is pretty odd that you are not getting traffic from Bing and Yahoo, as general SEO work for Google should also effect other search engines.  The first thing you should do is make sure that your site has been submitted to both and they have the location of your sitemap and robots.

Best practice you should put the canonical tag onto the main page as well as the other pages.

Yes, I have a lot of experience with GTM.  It is designed so that anyone can add marketing tags onto a site without needing to involve your developer.  It is a very powerful tool that you can use to define very specific circumstances with to consider conversions to measure external marketing such as banner ads etc. with.

The function of the meta description is to provide a snippet for the search engine to display, it is not a form of masking, that site is using the meta description for its intended purpose.

http://www.wordstream.com/meta-description

Look at your press releases how you look at content, if your press release is worth while and either ads value or answers a question then it can be useful as it will be picked up by relevant sites and used properly.  If you are just putting out press releases across sites which are spammy, then as Ryan said, they will not be helpful, or even have a negative effect.

Also, your Google traffic and your Adwords will never line up perfectly due to the difference of how each is tracked.

Google has gone into the differences here:  https://support.google.com/analytics/answer/1034383?hl=en

If your duplicate pages are behind a log-in you will be fine as content behind it cannot be seen by search engines. You should also block your logged in pages using your robots.txt.

You may want to be careful keeping pages live that are basically useless to visitors, if the ads are expired and it makes people leave your site (bounce) it will hurt your entire site, not just those pages.

My thoughts are instead of worrying about what is best for Google, think of what will give a user the best experience and go with that.  While it is nice to have a lot of pages index, if by the time they get to Google they are gone, what good does that do a visitor who was searching for a specific term that your site no longer offers? They are much more likely to leave which will effect your whole site negativity as bounces from search go up.

Are you sure that the URL's are the same, even small changes to URLs can effect your site.  Look at your Google Analytics to see if there is a change at the page name level and when it occurred.  This could also be a coincidence but doubtful) check your GWT to make sure that you have not been penalized at all.  Lastly, while your URL may not have changed, did the page layout and/or information on each page remain the same or was it changed?

Sorry to bare the bad news, but without being able to set up redirects you are out of everything you had besides the actual content that you can put back up.  All of your link power is gone, you won't be able to get any historic, in general you are starting from scratch but with (hopefully) a decent amount of already tried and true content.