Situation 1
I think this might be one of those cases when you have to go into the head of a player - which is always a case for trouble, and you have to decide if the fumble was intentional, because the definition of fumble (NFHS 4-21) is an "accidental loss of player control," where the keyword is "accidental." Now, if a defensive player is there and you think he might have caused the fumble in anyway, I would probably reward that defensive player and call a travel.
Situation 2
I'd go with situation 1
This reminds me of another I'd like to throw in that I've been debating for a while, hopefully cford won't mind
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Situation 3
Last season a kid was dribbling, he clearly (both feet) went out of bounds, but the ball stayed in bounds. He then came back in bounds, established both feet in bounds, then continued his dribble. My partner and I considered it legal. Were we right?