"In this case" there is no provision in the contract which says Facebook needs to keep Oculus accounts separate, so there's nothing to sue over. If that provision were in the contract, then yes I'd expect a lawsuit. Or at least some form of arbitration or settlement. Ideally the contract itself would specify what happens if that condition is violated.
I don't think that is legally feasible but I don't have enough experience in this. He sold the company, aka shares. The contract should determine how the sale happen but afterward, it's afterward.
A contract needs to be legal, and legal means what the law allows in the context. Does the law allow putting such provisions? I've been burnt by this in a rental agreement. Think about it this way: if we have a contract between both of us, where you agree that I'm going to kill you, I'm still going to jail. Having a contract doesn't make killing legal. This also applies to the rest of contracts. The provisions need to respect the law.
But the guy didn't have a contract, sold a patent-heavy company for $3bn (probably an army of lawyers involved) that netted him around $700mn. I'd just call this saving face.
