Never to kill the animal. NEVER.
Just use a sleeping dart in the future.
Not an option. The drug takes time to take effect, during which time the gorilla would have been further agitated by being shot by the dart. Doses aren't absolute. In his agitated state, the drug may not have worked at all.
All of the people I have seen interviewed who are professional animal handlers have said no to the idea.
The gorilla could have passed out in the moat and drowned. The gorilla could have passed out in the moat on top of the boy and they both would have drowned.