Are you assuming the masternodes have a majority of the PoW? Masternodes are orthogonal to miners.
No, I understand that. The problem (as I see it) is the masternode's quorum can be formed, scrapped and then re-formed with a conflicting double spend transaction all before their votes get embedded into the blockchain by the POW miners.
Could you unpack that for me? I didn't follow your math thought.
InstantX required the funds to be presigned over to masternode. I believe the idea in Evolution is all UTXO are eligible to spend through quorums without any presigning.
Last I looked, you needed to lock some collateral away to become a masternode, so acquiring N masternodes has a constant cost, as does acquiring a majority of them with which to perform the quorum attack described above. In regular POW chains, an <50% hash power attacker attempting to double spend has a longest chain production cost which is super linear in the number of blocks.