1) RAI, almost certainly only 1 allowed. However, there's nothing that states you can't use the ability multiple times, so go nuts. It's not like the class was well-balanced to begin with.
2) The ability specifies a creature you're possessing, which suggests that you must be possessing them for them to benefit. On the other hand, the bit about "as long as the fiend wants it to" might counteract that. Not completely sure.