So, let's assume it was in the actual contract. What is he going to do about it? Sue? Annul the deal?

No? Than it doesn't matter. The whole idea that because something is written into a contract that that automatically means that that his how things will be in the indefinite future is an illusion, and I've seen plenty of people burned that way. A contract only matters if (1) you are prepared to sue over it and (2) you will know what kind of remedy you want if you win the suit.

In this case the state of (1) is 'no' and the state of (2) doesn't matter because of (1).

"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.

At best an attempt at saving face.

Why the focus on what he would do? The promises were made to Oculus's current and potential customers. That's who would sue and it's fairly clear that they would want money... e.g. to go buy a HTC Vive.

I've upvoted you, since you are correct. I was absolutely amazed that you had been downvoted.