"MVCC can't stop two competing transactions debiting the same account pair (i.e., I have two linked accounts A & B with $100 in each and I launch two simultaneous transactions to transfer $150 away from both: afterwards, I'll end up with -$50 in both or some garbage data)."
That isn't true. In a pure MVCC world you could detect such issues, specifically that two trxs were opened and they both tried to update the same row. This would generate a conflict and one transaction could just get shot. Snapshot-isolation/MVCC describes this issue.
However what InnoDB/mysql and Clustrix actually do is use MVCC for read-write conflicts and two-phase-locking for write-write conflicts which trivially fixes your proposed problem.
That isn't true. In a pure MVCC world you could detect such issues, specifically that two trxs were opened and they both tried to update the same row. This would generate a conflict and one transaction could just get shot. Snapshot-isolation/MVCC describes this issue.
However what InnoDB/mysql and Clustrix actually do is use MVCC for read-write conflicts and two-phase-locking for write-write conflicts which trivially fixes your proposed problem.