“official” from the Haiku or HaikuPorts team, and while retaining the PaleMoon name, yes.
If the PaleMoon team decides to make an official release in their own way, they certainly can do so.
Not really, the goal of PaleMoon is to somehow keep alive an older version of Firefox. The code for current Firefox has probably been fully rewritten 3 times since then. And it’s not exactly a friendly fork.
Probably this blogpost: XUL Layout is gone | Words
However, I think libxul contains much more than just the XUL layout engine, in fact it is pretty much all of the Firefox web engine internals, if I understand correctly? The naming is a bit confusing if that’s the case…