It’s not very often you get a chance to take down a currency.

In 30 years from now you’ll be able to brag about how back in your day you took down entire cryptocurrencies with 51% attacks.

51% attacks are quite possibly wire fraud so I would refrain from bragging about them. More accurately, trying to double spend the coins is wire fraud.

Not if you have the consent of the counter-parties (or are the counter-parties.) It might be against some other law to manipulate the price like that, though.

Which government will prosecute?

The person they are swindling can go through their own authorities, and if the authorities in the criminal's country are alerted to his activities, they can go after them as well.

Theft and fraud don't take on different meanings just because the currency is digital.

So basically it sounds like you'll never get caught.

In 30 years no one will use crypto and people that invested in crypto will be laughed at.