If Nuka states himself in the movie that the hyenas ran off then that's most likely what happened. After all, it was in the script. You can't really deny a fact from that. It's not told differently elsewhere. (Officially. There've been all sorts of fan made theories.)

Not known. Never made it to the second movie.

No one knows for sure, but I believe it's been established that they ran off. In TLK2, Nuka says that the Elephant Graveyard is 'even creepier since the hyenas ran off.' So, it's up to you to decide, but I'll go off of an almost proven fact.