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Roman has covered most of the bases with his answer, so I won't retread old ground! But one thing I will note - my understanding is that with mobile-first indexing, content which is default-collapsed (to minimize clutter) won't be discounted. So if there is content you want to have on the site but the long-form nature is making the mobile experience feel cluttered, consider including it in expandable accordion style sections or similar. I would not recommend leaving it out altogether as Googlebot may no longer crawl your desktop site at all and all that content you add to the desktop site only won't give you any benefit.

Hi Ross, Several years ago we used to use Google+ with a mediocre level of engagement, but today it is mostly a dead social site. Google has even begun deleted profiles that are unused and appears to be showing signs of dissolving it completely in the near future. In my experience, I would advise you make sure you have a filled out profile on it (usually it will be tied to your Google My Business account), but then not invest a ton of time with posts after it is created.

Hey, Thanks for reaching out to us! Unfortunately this is not something that is possible within Mozbar - it sounds like it could be a great idea though! Feel free to reach out to help@moz.com if you have any other questions Eli

I certainly wouldn't use 2 sets of images. I typically use a framework like bootstrap or materialize and would just use the image class fluid. However when I'm working on a clients and they have some super annoying theme that I don't want to add any unnecessary additional plugins, or if the framework would cause any complications i'll just use the responsive CSS rules of max and min width and specify different optimized images at certain breakpoints: https://www.w3schools.com/css/css_rwd_mediaqueries.asp

Thank you. Will have a go at Google News - not sure we fit the criteria but can't hurt to try, eh??