> Serious question - could they actually nationalize starlink?

Yeas, why not?

> Suppose starlink takes their C&C keys and builds a control center in some other country

That won't happen. It is obviously better to be fully legal billionaire (even if the government take away 99% of his assets) in the USA than to be in some other country (which can in any time change it's mind), wanted for treason by the USA. Not even close.