The decay and poison effects have not changed from dom 1, when the battle is over the remaining poison damage is dealt to poisoned units. Non regenerating decayed units die, units that regenerate sufficent amounts survive. There is MR on decay.
I think that with enough HP it's possible to survive decay even without regeneration, but I'm not sure. I remember seeing one instance of surviving decay, but I forgot whether it was a regenerating unit or not.
I don't think so. At least I had a Dragon with 152 hit points left at the end of a battle go "poof" - which is why I remembered where to find that quote from Johan...