Android Auto: Google Play Music So this seems like an obvious "yes, of course it has to" but I can't find anything official that will commit to it.

Are all Android Auto consoles guaranteed to support Google Play Music, since it's an Android Auto app?

Perhaps more broadly: When would an Android Auto app not be supported by a given Android Auto console?
Android Auto is just a display interface app on your head unit. It doesn't control what types of Android apps are allowed, and it doesn't actually run the app itself. That's all done by the smartphone connecting to the head unit.

If you can install the app on your smartphone and it supports Android Auto, it'll be there. Thus the only time Google Play Music won't be supported is if your smartphone doesn't support it.