Why are there two Berens? Because Beren the Rider, husband of Redalda and king of the Berenethtelli tribe, is established to exist in later Glorantha. So if you win the game (and import that victory to later ones), Beren the Rider is your Beren the Tall; if you don't, a Beren still has to exist, and that's what Beren the Swift is for. (Of course, it's possible to lose badly enough that Redalda marries neither Beren, but that situation probably falls under Cutting Off the Branches.)
Ayvtu becomes unavailable to lead missions while pregnant, which does make sense... except that this never happens for any other female noble. Either none of them ever get pregnant (which seems unlikely, given the society) or Ayvtu the fire-warrior is actually the most cautious woman in your clan. Of course, having half of your nobles occasionally clanbound without warning would be extremely frustrating, so this probably falls under Acceptable Breaks from Reality.
