Musk's net worth his highly volatile. Telsa is trading at pretty high 200 P/e, one bad quarter could crater the stock price. Space X is private, which carries its own issues. Add to that the risk to these assets if Musk dies. Or hell, even it Musk merely "spooks" investors with his flamboyance.
No matter how volatile his worth is, it's just around a tens of the said worth and there's no way it loses 90% of its value overnight, even if TSLA's valuation is arguably nonsensical.
If he has it. At standard margin rates, he'd need to put up ~$140 billion in TSLA stock to cover his TWTR offer. How much can TSLA slide before he doesn't have that. Keep in mind, private companies (SpaceX, Twitter in this hypothetical) cannot be used for margin purposes.
You can still sell the rest of the stock and cover a majority of the loan. I still fail to see why you need way more money than the loan amount to pay it off.
This is a insanely risky vanity project.