[Karellen]

it seems tmdb has purged/moved a bunch of images that I was referencing....BOOOO

Yes. Many users have reported the same issue but it is out of our hands. TheMovieDB admins have confirmed there was a mass purge of artwork, but have refused to discuss the reason behind the purge. Even some friendly mods at the site are not privy to the reason.

But ok, now what do I do? 

Not a lot of options when the artwork has been removed from the provider and you do not have local artwork. Two options...

1. Install Artwork Beef addon and configure it to run through your library and add missing artwork. There is also an option to save the artwork locally, so you don't have that problem again in future.
2. Remove your sources one at a time, answer Yes when asked to Remove from Library. Then run a Clean Library. Now re-scrape that Source back into the library.

Should all my frontends be sharing a networked metadata store?

You can, but the main benefit of that is sharing play counts, watched status etc. If that does not worry you, then keep them separate.

if I took one of the local metadata stores from one of my other frontends, put it on the network, and then pointed the new one to that, would it just all magically work and my problems would be over?

Probably not, but I am not sure of your exact setup. Keep in mind that Paths saved in the database are relative. So when moving to a different machine that has a different address, they most likely will not work.

Bonus points: I'm switching to a new theme that makes use of backgrounds ("fanart"?), and most of my content does not have that populated...is there a way to populate fanart for existing library items?

Artwork Beef

I don't really understand how metadata works in the system as a whole, or how to manipulate it to my will

NFO files, or install the Metadata Editor add-on... https://kodi.tv/addon/program-add-ons-sc...ata-editor