Can a 51% Attack Steal Existing Private Keys or User Funds Directly from Wallets?

No, a 51% attack cannot steal private keys or funds from wallets that have not been moved by their owner. Private keys are the cryptographic secret that authorizes the spending of funds, and they are never broadcast to the network or stored on the blockchain itself.

A 51% attack only allows the attacker to manipulate the history of transactions, specifically their own. They can reverse their own recent transactions to double-spend coins, but they cannot create fake transactions from other users' wallets as they do not have the corresponding private keys to sign them.