If I ready a character that has just committed to the quest (e.g. with a mount or with Unexpected courage), can he or she quest a second time? What if I quest Dain and ready him, does his bonus apply to other questing dwarf characters and to his own quest will?

Thanks,

Brumbar

Nope, characters are either committed to the quest or not. You can't commit them a second time even if they are unexhausted. You can however then defend or attack with them as well =)

Yes if you quest with Dain and then ready him, he will ad his bonus to all dwarves including himself.