Hello! The card used for eliminated is NOT also eliminating, only the card that was already in play. You cannot eliminate a card of the same rank, only cards of a lower Rank (with Aces obviously being counted as alternatively low or high cards--they can't eliminate each other, though!)
Your hack sounds interesting! I'll have to give it a shot at some point. The reason this game only uses a few cards out of the deck is that I'd originally envisioned it as a game with all unique cards (as opposed to playing cards) and I was nervous about printing costs. Obviously though, there's no reason to hold onto that for a game that DOES use playing cards, so by all means!