An sp-first person could be neurotic about personal safety or well-being because it doesn't come naturally to them so they have to pay attention to get it right, whereas an sp-last may be naturally good at that sort of thing and can essentially ignore such things and still be fine.
Again, yes, we're neurotic in the first instinct, but I also think the "neuroticism" is more a manifestation of that hyper-awareness of it. A sp-first person will still have a greater awareness and innate knowledge (They're instincts, right?) of sp related concerns and be neurotic about it than a sp-last person. A sp-last person has to work on sp because it
doesn't come naturally to them. They ignore it, not because they're naturally good at it, but because they just don't care about it, and if you don't care about it, how can you be neurotic in it? The way you formulate how the instinct appears in the last position seems counter-intuitive to me.