i don't know if there is anyone here who has solved one of Satoshi's puzzles or not.
but let us
suppose we can find the private key of a puzzle.
will this key be enough to import the btc in a new wallet (like Electrum)??
or, in addition to the private key, will a two-step verification or multi-signature or something like this, be required?
It depends on which puzzle you have solved, if it's 66, 67 then you won't succeed with electrum, you'd need to either have a bot that automatically double spends the tx or talk with a large mining pool to privately include your tx in a block, because once 66 or 67 start moving on chain and having their public keys revealed, it would take seconds for looters to brute force the public key for the private key just to double spend your transaction and loot for free.
But if it's 130, 135 etc, go ahead and use electrum with Tor.
There is no multi signature etc.