The thing about instincts is certainly an interesting topic and, as I've told OP in another thread, one that I've personally never given any thought.
There is a contradiction in your reasoning though, if that's okay for me to say: you claim that wanting to die is unnatural (let's just assume it is, for the sake of the argument), but we do have to acknowledge that there's lots of exceptions to this. Thousands of people end their lives everyday, yet this does not seem to change your point of view on this being an unnatural thing (which is fine, just bear with me)
At the same time you'll argue against the existence of primal instincts by the example of women abandoning their children.
So if you've previously established that exceptions are indeed a real thing (like people wanting to die, even if it's deemed unnatural), why would such an exception not be possible for mothers abandoning their children? Isn't it by this logic also possible that instincts are very much real but there's exceptions to it as well? Like SOME mothers abandoning their kids and SOME people wanting to die, when in both cases, the vast majority won't? This would also be further confirmed by different people experiencing different levels of SI. It's stronger for some and almost non-existent for others.
I guess my point is: something being unnatural does not invalidate it in any way, shape or form. Being gay is also not exactly helping human reproduction, but this does not mean homosexuality isn't real (Sorry if this is kind of a sensitive example, but I feel like it's a good one)
Still, I'll give this some more thought tonight. Luckily we can all have our own opinions. Either way, I 100% agree that instinct, fear and needs are three totally different things that are often treated interchangably.