If you re-enlist with a bonus, but during your contract you get med boarded, will you have to pay the bonus back?

No...the Army will not seek bonus re-payment when involuntarily medically separated.

Highly doubt it because it's not something you typically have control over. Popping on a piss test is one thing. Getting hurt is a completely different thing. And yes, I do know someone who got a reenlistment bonus... and then pissed it away the next week when he popped on a piss test.

Very rarely, but probably not.
If the injuries are the result of some form of negligence on your part - a finding in a Line of Duty, you need to be concerned. Otherwise no.