@Pandora Apparently Pax did think Pravus had a vassal or two on the mainland, so when she wrote the post (Part 1 and Part 2) it was meant to apply to any current mainland Pravus vassals as well. But that's actually moot, since all of Pravus's vassals are on their island chain already.
There has been at least one PC house that chose to stay with a grand-liege upon elevation (Stormbreak left Kennex's chain to stay with Grimhall when Kennex was elevated). I couldn't say if the reason for that was "extraordinarily good" or just cause they felt like it. I kind of just assume players will go where they want to play.