When an InstantX lock is achieved (takes a couple of seconds usually) and broadcasted (and the merchant will see in his wallet the tx got IX comfirmation), how do you propose to reverse that?
Read the thread and find out.
If you own a majority of masternodes why would you do something that undermines your wealth? The 1000 coin collateral is there for a reason and that is to have an incentive for the nodes to act in the best interest of the network
Yes, this is the common rhetoric we hear from POS stake disciples as well. Despite the fact that you can short coins on exchanges these days, this argument does nothing to dissuade the irrational attacker.