I account for your sense of idealism through your 1 wing. That's more accurate since idealism is a worldview. Instinctual variants don't contain world-views but simply cast us with an overlay of general behaviors found all throughout the animal kingdom. Instinctual variants are hardest for people to accurately self-type because of the fact they are biologically rooted drives operating entirely outside our conscious awareness.
I use your core type (9) to explain your drive to merge.
Merging is the area of fixation for type nine.
I don't see you as soft or diffusive enough for a sx-first. A sx-first nine is especially diffusive. There are times when you lock into another member in a way that seems to pull you out of your own skin but its for short bouts. It's like the SP-Wall comes up for a bit but then closes down abruptly and you snap back into your 'underground layer' once again individuated. I find this to be very typical of sp/sx.
As for your socionics type, Ne and Ti show up prominently. If you were an IEI, Te would be your vulnerable function. There is no way that Te is your vulnerable function....there's too strong a tint of objective reasoning processes in your debates with other members. Also see the exchange you had in the Halloween movies thread with Totenkindly (Ti/Ne). Your posts were prototypical of Ne/Ti and almost similar in information processing as Totenkindly. The difference between you and LII is that you advocate more passionately for your ideas. It pertains to what Jung talks about Ne's dramatic fire and more zealous style of presentation, as follows:
"The intuitive's morality is governed neither by thinking nor by feeling; he has his own characteristic morality, which consists in a loyalty to his vision and in voluntary submission to its authority."
"He brings his vision to life, he presents it convincingly and with dramatic fire, he embodies it, so to speak. But this is not play-acting, it is a kind of fate."
Back to your lead instinctual variant, I don't see anything in what you wrote as indicative of sexual first.
Nonetheless, your reaction to getting burnt strongly indicates SP-first. Getting burnt continually can cause the lead variant to flip. I don't think I ever talked about Flipped Instinctual Variants on here before. Under stress, the flipped side of the sexual variant turns to provocation. The flipped side of the self-preservation variant turns to cutting ties and hoarding resources (including the self). Becoming more cautious and reserved as you described is consistent with the SP response.