This thread broke my motivation to keep searching. I'm thinking of pausing the search if there isn't any definitive solution.
Attackers are unlikely to succeed if you make a transaction with the highest fee and non-RBF.
Bur if you feel you're wasting time an money on the puzzle 64 just ignore it! and focus on the puzzle 120 which you are more likely to find and there is almost no risk of attack for it.