1. According to this developers.google.com/identity/sign-in/web it seems to just need a few lines of code. Considering in C3 we have access to HTML/Iframes/Javascript I think it should be possible, but I haven't tried this way. I have made a login with firebase though. (See point 3)
2. I'm not aware that anything major changed in that regard with C3. You still have stuff like the Anchor behavior that are supposed to be used for that. The new HTML plugin might be of great use I think, for displaying certain stuff like lists responsive.
construct.net/en/make-games/manuals/construct-3/plugin-reference/html-element
3. I see two options: Offer a backup button that downloads the safe file to the device outside of local storage. Or have a database online that (also) stores the saves in case the device explodes (or something like that) You could look into google firebase for that which I think would mesh well with the google login. Or you can run your own server (either way will cost money)
4. Short answer, yes: construct.net/en/make-games/manuals/construct-3/plugin-reference/mobile-iap But read through carefully since there's some caveats to this.
5. If it supports javascript, I'd say most likely yes. I haven't tried though.