So if it goes bankrupt what happens ? None of his assets aside Twitter gets seized ?

If Twitter fails to make payments on its debt then Morgan Stanley will become the proud new owner of Twitter. It is similar in structure to a mortgage on a house.

That seems like a really stupid loan to make then. If Twitter fails, it becomes mostly worthless, much like MySpace today. Why would a bank make such a loan? (It does seem like bankers are pretty stupid these days though, or just don't care because it's not their money.)

They set an interest rate in line with the risk that they are taking on. It seems like a pretty unlikely scenario where Twitter is worth nothing, if it goes bankrupt there can probably be a fairly sizable sale to recoup some value.

Allowing so long for the deal to close may have been a mistake due to what happened in the market between when they agreed to terms and when the deal closed.

He already paid 20B+ of his own money. Debt is only part of financing, AFAIK ~13B.