Or....he structured it as "let me borrow 60% of X, and here as some shares worth X as collateral.". Aka synthetically manufactured selling shared without actually doing it in order to fund his lifestyle.

Depending on the advance rate (the 60% in this case), someone will do that deal.

To take it to the absurd. If Elon wanted to fund his $35M daily party habit in the comment above, and he said to a bank "here is 3 billion in stock as pledged collateral which you can sell when the value goes to $500M but there's no recourse. Cool? Cool. " You would do that deal.

So the margin call concept does not seem realistic.