For what is essentially a turn based combat system there definitely needs to be a fixed turn order. In my mind it should be  apply damage -> apply heals -> test if dead. -> next creep. 

Over healing is fine imo as a mechanic it essentially balances itself. So you can pre-heal a creep that effectively loses you a tick of heal if there is DPS swap. Assuming the difference between heals and damage is close clever target swapping will still break a creep formations tank. All pre healing does is make it harder to alpha a creep. On top of this you would still need some way to workout if a creep makes a sensible target in the first place.