Re: 64. Can the Transfer of the Inventor of the Puzzle Be Manipulated?
Yeah probably possible. On the other hand, why would you listen for potentially few years on the nodes with ready to crack hardware to get 1.2 btc to make a potentially successful double spent attack?
1.2 BTC is currently worth close to $75k. Someone could potentially create a program that listens for a transaction that spends one of a set of outputs, rents a VPS and GPUs on GCS, and executes a script that will find the private key, and create a competing transaction.
Where did this value of 1.2 BTC come from? 
The Puzzle #64 address only has a balance of 0.64020585 BTC
Or is the 1.2 BTC the total value of all the "prizes" that have been claimed so far... and someone is theorising that an attacker may have attempted to setup a monitoring rig to try and steal all the prizes?
I haven’t looked at the OPs math.
The Tesla V100 costs about $0.21 per 5 minutes to rent from GCS. I don’t know if google has the capacity, but someone could rent ~357k GPUs for 5 minutes for $75k. I don’t know if this would be sufficient to find the private key. You can rent ~176k for 5 minutes for half that.
If you can figure out how to quickly calculate addresses on a TPU (ASIC that is designed for matrix multiplication), you can rent ~880k TPUs for 5 minutes for $75k. If performing calculations that TPUs are optimized for, the efficiency of a TPU is at least a factor of 10 more efficient than a GPU. Although I don’t know if google will allow you to scale that many TPUs.
Given that the private key range is known for each puzzle, I am not sure I understand the advantage that someone will have once the public key is known.