Can the health of the emitter, and various other units be changed? If so then how?
For PRPL cores yes.
For the rest, someone that's better than me at the coding (V, PF, K) will have to answer.
I haven't seen anything that makes it easy though.
In theory, if you have the UID of an emitter, you can use <-uid <-amt SetUnitMaxHealth. I don't know if that will actually take, but it's a starting place. I don't think there's a way to change the healing rate, though.
Does the emitter health start at one?