Halsin and the Companions not "staying put" reminds me of Wiz 7. Anyone else remember that game? Crusaders of the Dark Savant. It had this great feature I haven't really seen since (if you have, please enlighten me). The NPCs in Wiz 7 would roam around the game, fighting each other and hunting for McGuffin Map pieces. You could run into them randomly or check on them with a Locate Person spell. In BG3 the fact that companions can get bored of waiting for you and go about their business really adds to replayability and immersion. It would be interesting if some of them popped back up after Act 1, possibly as antagonists or third party actors.