Excrementum sancti

Interesting thoughts.

Yes I'd agree that overgeneralization sustains avoidance behaviour which short term can be with your cost/benefit reults. On the other hand it hinders more diverse behaviour long term and that would be (or at least could) a negative evolutionary fit. Which would not make it (ptsd) a positive evolutionary trait but just something where it stayed in spite evolutionary pressure.

The intrusive thoughts are often seen as refreshing the actuality of the trauma, but not as purpose but more as a unconscious attempt of integrating the emotional "short-curcuit" of a experience

And being more irritable and showing a faster fight flight reaction helps with forceful intercourse how?

I believe there is more to selection than surviving. If you want to reproduce, you have to be able to function in a community which is very hard while suffering a mental illness (in a pre-modern community that would be). Also, one would probably not be a preferred mating partner.

I would tend to see it as a disfunctional process that may in some cases be useful but in other cases as a evolutionary disadvantage.

I'd look at it like a mental scar or wound, that sometimes can have a good and healthy outcome and sometimes fester and become infected.

This would explain the evolutionary usefulness of the hypervigilance part, but not the intrusive thinking (nightmares, flashbacks, etc) that are signs of ptsd. Also, there is a tendency to show an overgeneralization of dangerous situations that leads to a lot of avoidance behaviour - that in the long run could have been a negative evolutionary factor