5. I double spend the payment in another instant X transaction which also happens to hit my masternodes
Impossible. In order for an InstantX transaction to be locked, a majority of the eligible masternodes for that UTXO must sign the lock. Thus there are not enough remaining eligible masternodes to sign the same UTXO again. (If Dash doesn't actually work this way I described, then it should be fixed to work this way.)
What defines eligibility?
Indeed, why wouldn't the same set of masternodes be eligible to sign the same UTXO? Unless there is some magical, permanent evidence that I have already signed the previous UTXO, the exact same set of nodes will be eligible to sign it multiple times within any deterministic eligibility window.