also I think (fairly new thought) that HD keys that were reused could be soft-forked to require a Zero Knowledge proof of knowledge of the chain code and master even if the coin private key was public information. (and soft-fork made not be spendable with direct ECDSA.).
I wonder how something like this could work considering the fact that any information provided based on hashes that could reproduce the keys could be duplicated by the other parties that are trying to steal the same coins.
But I am not sure how P2PK worked. Has the public key changed every time for early wallets?
The same as any other output script but instead of using hash of public key you use the same public key. It could be reused or the wallet could produce a new pubkey for every new payment (which was the default).