1. In order for the zep to land on the mountain to pickup the princess, you have to have the king on-board. Getting him there is the tricky part. Make sure you have that quest of locating her. Did you get into the staring contest?
That was a cool one
But you have to go back to get the King on-board the zep, then you can land. Might be a bit tricky landing cause the area is small, but keep at it.
2. the vanishing and moving one? yep. keep going back and forth to the room. eventually it'll show up in a place you can access to get it.
3. now as for the hydra, if you challenged him straight off the bat or teased him he couldn't fight, then he'd join, but you will lose out on his naming that pear. Once you talk to him and left him there without him joining the party, then you won't get another chance to change his mind and he can name that pear for you.