> Reversing charges is a different point, which could be accomplished pretty simply with 2-of-3 sigs if there's market demand for it.

You can’t reverse transactions. Once validated and included in a block, the transactions are forever, which is of course one of the main appeals of cryptocurrency in the first place. You would have to get the fraudster to send the money back (potentially covering gas fees) or else there is literally no other way without their signing key (also one of the key appeals of cryptocurrency).