Both good points, aye.
It doesn't say it outright, but in Book 1, here, in the fourth-last paragraph, Parson writes that "Royal empires split off sometimes into new sides", while explaining the whole concept of Royalty - which implies that non-royal empires don't, otherwise that is a very strange thing to say.
Of course, Book 1 was rather a while ago. It's entirely possible Rob has changed his opinion on that since.
EDIT: Also, I guess saying that non-royals cannot found new sides was inaccurate - this merely implies they cannot split off from their parent side to form a new side, but if a non-royal heir found himself a barbarian after his side was crushed, s/he would most certainly be able to found a new one. However, given that we were talking about this in the context of splitting off a new side, that's rather irrelevant here.