Thanks for the feedback!
I think the issue is that we relied on people's "meta knowledge" of other games too much. We used wording very similar to that used in card games, with a similar structure for similar effects. If the card doesn't state that the effect only affects "active" creatures, it affects the base species. "+1 strength on melee kill" is an effect that gets added to the base creature, meaning if you spawn one it then has this effect. To describe this in a more precise/less ambiguous way, the text would have to be much longer, which is a clear design trade-off. It's tricky... We definitely could've worked and on the card descriptions more though.
Showing health bars is something we discussed and decided against if for aesthetic reasons (to not clutter the screen when there are 100 fish and 30 enemies later in the game). With more time, we definitely would've made it toggle-able.