This is where all discussion of WC3 mapmaking should be held. This forum exists particularly for mapmaking needs. If you have a question on how something works or have need for some code review, this is the place to put it.
Events: -Unit is issued order
Conditions: -Order = immolation_off
Actions: -Stuff
will the trigger still activate if the unit that was issued the order had no mana and therefore could not actually turn on the ability? Or will I have to check if the unit had enough mana myself?
I don't think it will count as actually ordering the unit to do that because no mana overrides the order and stops you. I suppose it's similar to the way if you don't have enough mana to cast a spell the triggered effects won't take place.
So to answer your question, you'll probably have to check mana.