People will watch the mempool, check for solved puzzle publickey, solve it quickly with pollard kangaroo and create a new transaction with higher fee.
Someone can already do this and pay a miner privately to include their competing transaction over the one in the mempool. Full RBF makes it easier, but it is already possible.
If you don't want the public key revealed, then you could also do the same and pay a miner privately to include your transaction.
I didn't understand what you mean exactly but when you send a transaction just set it to no RBF, and that's it. If someone attempts to send the coins again, it'll be flagged as a double spend and nodes won't broadcast it to miners
We are specifically talking about full RBF, not opt-in RBF.