Cancellation is a last resort.
This is actually American law, neither English nor Roman. While it is derived from English common law, it has an even stronger bias against specific performance (and in fact bright-line prohibits some which would be allowed in the earlier law from which it evolved, because of the Constitutional prohibition on involuntary servitude.)
However, I have always maintained that making the plaintiff whole should bias toward specific performance. At least that's what I gathered from law classes. In many enterprise partnerships, the specific arrangements are core to the business structures. For example, Bob and Alice agreed to be partners in a millions-dollar business. Bob suddenly kicked Alice out without a valid reason, breaching the contract. Of course, Alice's main remedy should be to be back in the business, not receiving monetary damage that is not just difficult to measure, but also not in Alice's mind or best interest at all.
It was looking like he would lose and the courts would force the sale, but the case was settled without a judgement by Elon fulfilling his initial obligation of buying the website.
No, he couldn't, the widely discussed breakup fee in the contract was a payment if the merger could not be completed for specific reasons outside of Musk’s control.
It wasn’t a choice Musk was able to opt into.
OTOH, IIRC, he technically wasn't forced to because he completed the transaction voluntarily during a pause in the court proceedings after it was widely viewed as clear that he would lose and be forced to complete the deal.