Let's see if I can clarify myself... I think you may be doing something very simmilar of what you're accusing me of doing - the whole concept of triggers isn't named in the rulebook or FAQ, and not referenced in any ability, but without them, the game doesn't work. No matter which overarching framework we assume to exist, we have to assume something. I just wanted to illustrate an alternate fundamental perspective of the rules and the wording on the cards that functions in a way that didn't rely upon the words as triggers but words as timing windows.
The part I bolded here is, I think, the big difference between how I normally try to approach it, and what's going on here.
X-wing lacks deep and detailed timing rules and defined terminology. An awful lot of the foundation we use to figure out how abilities interact is based on extrapolation and reverse engineering.
But we (or at least I) do that to the minimal point possible. Without some sense of trigger timing, the game just doesn't work. Without some limitation on the number of times you can activate an ability in response to a trigger, the game just doesn't work. So we accept that.
The problem with pretty much every argument I see here (including the "timing window") is that while they may be in the same realm of reverse engineering the rules, it's way beyond any minimum requirement. Does anything break if we don't have a concept of cost, or distinguish how abilities work based on that? No - everything works just fine. Opportunist follows the same once-per-trigger that we expect of every other ability in the game. It still works, just not the way people want it to work.
Which is the other reason I'm not at all on board with the direction things are going here. It seems to me that this is not an attempt to figure out how Opportunist works - it's an attempt to justify how people want it to work. Nobody brought this up in regards to Elusiveness, because Elusiveness sucks. But a shiny new card that could give you +3 attack dice? Time to fire it up and find a way to make that work! The evolving thought process illustrates that perfectly - first it was cost, then cost didn't exist so it was choice, then it was whether it was a ship ability or upgrade ability, then it was maybe just EPT that was different, and now we're back to cost, which has the advantage of not being disprovable because it's never mentioned. That's not trying to figure out how the ability works, it's trying to make the ability work the way you want.
Is it possible that FFG has an inherent concept of cost that they've never articulated through the rules, that abilities which use that concept of cost are triggered and handled differently than pretty much every other ability in the game, and that they'll rule as people here seem to want? Probably depends on whether or not it's Tuesday. But based on every precedent we have in the game right now, every understanding of how every other ability works, Opportunist triggers once and has one opportunity (pun intended) to do its thing. Who the heck knows what FFG will do... but until they do, we should evaluate it based on previous precedent and other abilities, not try and invent a new distinction with a purpose.