Really, Mana cheat just shouldn’t be enabling plays far ahead of reasonable answers to them.
Almost no one has turn 2 5 damage removal spells, so double gnolls shouldn’t really be able to hit the board on turn 2 even on a high roll.
3 damage AoEs are usually turn 4-5 ish? So both halves of sunken vessel shouldn’t be happening on turn 2-3.
Mana cheat isn’t inherently a problem, but it should have limits on how lucky someone can get with it.