If Donovan goes to the Magic, can recruits that he signed (Jai Lucas) get out of their commitments?

Only if the school they signed with releases them and they usually do with the exception of a very few cases. So I would say that Jai Lucas will try to get out of his commitment and if he does Florida will release him. Just guessing as to what I think will happen. I predicted Donovan would leave Florida and I was right there, but for Florida's sake I hope Jai Lucas stays. But I truly look for him to get released and try to opt for his second choice which was Kentucky where his buddy Patrick Patterson went.

Yes. Normally it's binding, but they can apply for an exemption in certain circumstances, such as the coach leaving, and get out of it. The school can decline to release the player from their commitment, but they'd be stupid to, as it would give them a bad rap with future recruits.

Only if the university agrees to it. Usually universities do... but when Huggins left Kansas State they did not let Michael Beasley leave.

I'm pretty sure you can. It's not a legal contract.