It got stolen from the original solver by someone who noticed the transaction and posted a bigger fee

By what means might you prove or disprove this theory?

Having the the solver post all 2^66 -2 incorrect private keys would prove that they solved it fairly and had no prior knowledge of the key.

You'd just need to download the 6,505,548 TB list of keys and re-derive the public key for each to check that they're valid; unfortunately it would take in the ballpark of a kiloyear of compute time assuming you have 3x RTX 3090s.