So to make my design really robust, the payees need to be listening so they can enforce the protocol. Remember I am making a micro-transaction coin, so the payees will be online often. And often is good enough. Because if the payees clients blacklist the 50+% adversary's chain for violating the protocol, then the adversary could have 99% of the PoW resources, but if they constantly lose a larger and larger share of the payees, then they honest network has forked away from the adversary and filtered it out.