I understand its also possible, although not through the official site, to play a given card at a lower level, particularly by bots.
I'd like clarification on this point. Given my understanding of the blockchain mechanics behind the game, I don't think anyone currently has the ability to play down a specific card.
However, perhaps what you're saying is that the current official site only registers your highlest level card for a given moster and ignores the rest. Programattic users however have the ability to submit a card that is lower level than another card of the same monster for battle. @yabapmatt is this the case?
My proposal is different from the current behavior available to programmatic users because it would allow temporarily leveling down a card for a specific battle rather than require the user to possess a low and high level card of the same monster.