Very interesting. The tradeoff mechanic was definitely noticeable, though I wasn't quite sure about the hearts mechanic's interaction with the number of people you had, or whether there was one at all. I do wish that enemies couldn't fire from "beyond" if they were already completely off-screen, if they weren't going to come back themselves. If you're doing a polished version it might be good to stop the progression once you die (I was sitting on the dead screen and then got a warning about having to fight a big enemy and there was the boss, even though I was dead already).
All in all great thematic addition to the jam. ^_^ Cheers.