AngshuDhar Critical only works while falling. I am sure of it. Go test yourself, take diamond axe, spawn 2 villagers and hit them. The first while falling and second while jumping up. You'll see the first takes first takes 2 hits and the second takes 3 hits. Always.
That's why Jump Criticals will never work as a standalone module, because it would require it to somehow predict when the player is gonna hit. They could, however, be incorporated in FightBot, KillAura etc. to jump and hit while falling.
As for your suggestion, it's still a packet critical and it still does something it shouldn't, so a decent anticheat will detect it easily. It will not fool people who know of the falling condition part either. It could fool new players though.