Is invincibility possible?

Say there was a unit (purely hypothetical situation! ) - we'll call it Unit X - that was unable to be harmed by unitcombat type Y. Is this basically the equivalent of making a promotion with <UnitPromotions_UnitCombatMods> with <Modifier> being 9999999999999999699, or does that still make Unit X take at least one point of damage? And would this cause Unit X to instantly be able to kill a Unit Y as well?

I'm fairly certain (not looking at the code right now) that even if the strengths are really lopsided there is still a possibility of the strong unit taking some damage. You would probably need to write some Lua to implement 'true' invincibility.