The attack I described for the Finney attack didn't even require conflicting InstantX announcements, rather the conflicting announcement of the transaction being spent on the block versus on the instantx permissioned masternode. Even if InstantX are prelocked to a specific masternode, it must be possible to unlock the funds back to the general UTXO otherwise that would be a risk of losing funds to a masternode that refuses to sign. So thus the Finney attack can just unlock the funds in that case to create the double-spend of the InstantX transaction (by unspending it). No matter how it is designed, it can be attacked. I don't even need to know which way it is designed. I can reason it is flawed in any way.
If the lock can't be acquired within 20 seconds iirc it will lapse. Can't find a link to back my memory though. And I don't know how and by whom it is canceled either.
Don't think only of the merchant (payee). Think of the consistency of the block chain.
It's starting to get too complex for me for now, I guess I'll need to draw a picture of a forking network to understand the implications. Thanks for being patient with me and trying to explain.