Realize that normally masternodes can't lie because it is deterministically determined which masternodes can lock which transactions. All the masternodes can do is sign the lock or not sign the lock.
Some masternodes could attempt to sign locks which they are not designated to sign, but the PoW block chain would never honor these locks and would not put them in the block chain. And payees can do this same verification before accepting the InstantX transaction.
Remember we are talking about 0 confirmations here, there is no POW evidence at this point, all that exists is mempool evidence. Payees might well be able to do that verification, but that's not in the protocol.
If I own a majority of masternodes, there is a greater than 50% chance that any one of my nodes will get picked by the deterministic selection process, so they can indeed lie with a high probability of success.