hamysho: is there any way to get your companion to leave your party to force them their XP to update? Just resting while in the castle doesn't seem to force the XP check
Is the only time XP will equalize near the end game or are there other times party members will "leave" and XP will update.
No. Some of them leave as a part of their quests, but most of them will have to wait till the house. Ones that leave at one point or another are:
Ocatvia and Reggie - during last parts of their quest
Tristian - after vordakai.
Maybe Amiri during her quest line, but I'm not so sure about that one.
hamysho: Is there a shield bash paladin that is half decent or are slayer and ranger Shield Bash (SB) builds far superior because of Sneak Attack damage (slayer) and the removal of Dex in 2H fighting styles (Ranger) far superior.
Paladin just doesn't have the feats for shield bashing. At least not unless you are willing to sacrifice everything for it, which is not a great idea. Paladin is either a 2h or an archer at his core as he just doesn't have the AC. Monk/pal can be on the frontline, but there is an elephant in the room called magus, as both saint and scion do that job better. And there are not that many evil enemies in the game. Still, monkadin is serviceable.