It is also entirely possible to get a second path any level after level 4. One of my more recent games I got the option of another path at level... 6 or 7 I think?

Of course, it was also one of those crappy playthroughs where all I was getting for level up choices were the really weak generic traits and spell levels, so I think the game assumed I wanted to become a Mage after "regretting my choice" as a warrior.

I've seen this too (playing on Challenging) - ALL the AI sovereigns get both the Warrior and Assassin paths (as early as level 4 in one case), in every game, as reliably as them all having more leather greaves than they could ever need. I've seen the multiple strength thing too, but not as reliably. It's always these two paths too, even for mage characters like Procipinee and Ceresa.

Might be, although in my case I tend to exit & restart the game after finishing a game because of the problem with custom sovereigns losing all their abilities, and it still seems to happen.

It does not only happen on "higher" difficulties. On normal, all AI sovereigns happily start with merchant, path of warrior and in some games, strength for inexplicable reasons.

My bad, Aus' quote explains what I was seeing. 

I actually normally don’t like getting multiple paths on a champion. When specialising a champion there are key traits you want to pick up, having two paths dilutes the selection and you end up picking things you aren’t really keen on. If I’m building a warrior or assassin the last thing I want is prodigy or summoning popping up. There are occasional exceptions of course, but generally I want to maximize my chance of getting the really good selections.