I'm not sure about the other stuff but I have used different party members, Gale for certain, to investigate the Book of the Dead. Not sure why you can't.
I can do that, but each attempt makes you start over from the beginning as if the earlier skill check hadn't been figured out already. It's like if there isn't one character who has all the necessary skills to complete a dialogue in your party, you're just SOL. Which is silly because it SEEMS like it should be so easy to just ask a companion for help.
This would also just be a good idea for the interaction scenes in general. When lockpicking for example I sometimes forget to have Astarion selected, so I need to back out of the dice screen, select Astarion, and then select 'lockpick' again. It would be so much better if you could just switch characters on the dice screen itself.
