I feel like having a single cell health is a little too great a level of abstraction to be either realistic, or engaging, gameplay-wise. Ideally, a cell should have one main health for the intergrity of its membrane, such that if that ever hits zero, the cell ruptures and dies, but internal organelles should also have their own health, and the cell can potentially die if enough of its vital organelles cease to function. And engulfing should be a completely separate process that doesn’t deal damage per se, but does kill the victim cell upon completion.
I have additionally been thinking about how to make predatory pili more reasonably balanced, and have come up with the following proposal: Make them work like the barbs that nematocysts (the organelle in the tentacle cells of jellyfish that sting) use. Simply bumping into another cell with a pilus won’t do a huge amount of damage to that cell’s membrane, but can still function as a limited deterrent against engulfing. If you want to use pili offensively, you need to arm them in a fashion similar to the way nematocysts wind themselves up. Arming pili costs a small amount of ATP and can stay armed for a limited time, but allows them to deal massive damage to the target cell’s membrane, and possibly some damage to internal organelles.