Since GPU farm is worthless, 71 has almost no chances of being solved this year. Maybe in next years, unless someone is very lucky or the price goes to 200k.
And about withdrawing the funds. Mara is the only option right now, either you trust it or not.
Once a transaction is confirmed by a miner, there should be no more funds in the source address that a bot or anyone else can resend at a higher fee, right?
Yeah, once Mara got one block mined the coins is safely moved at the same time. I've made a tutorial of how to withdraw the funds safely throught Mara. But in a nutshell: the coins remains in 71 till mara mines a block, until that the public key remains not exposed.
When you use public mempool, the transaction is shown all the time on the wallet's history, including mempool. When you do it through Mara, there will be any evidence that you started a transaction until Mara confirms it in a mined block.
Thtat's the main difference...
Do you think it's still possible to steal the response if we sign it with its symmetric private key? For example, look at the private key of Puzzle 69: