Abilities have an Ability ID, and a Text - Ability Order.
The Ability ID is a number that identifies the ability. If you have 2 abilities based on the same, they will both have the same Ability ID, and they both will be detected as "casted" even if you just used one of them. It's "buggy".
The same happens if two abilities have the same text order ID, but these can be changed. The order ID is the... "order" you give to the unit when you cast it. You order your unit to "attack" to "move" and to "thunderbolt" or "flamingarrow" or "wispharvest".
The only abilities that allows you to modify the Ability ID are Channel and Spellbook.
Warcraft has actions to cast most of the skills, but not all of them. You can give unit orders that aren't listed in the "Action" list using custom scripts.
Yes, you can do whatever you want with triggers.