Yes. F-droid allows that, you can have multiple repositories and ... a few of those even exist :)

I'm using that with 2.0 (or rather with F-droid Basic, but I noticed Basic was updated to the same that is described in article, but a while ago). It works just fine.

It's always been possible on F-Droid, I doubt they would change something like that.

Yes, that's how you can install an manage Molly (the open source Signal replacement). They run their own F-droid repo.

Of course. Don't even need F-Droid 2.0 for it. Fdroid 1 still supports multiple repos