Part A: If the apple maggot flies preference for the apples so much that it is equal between the two trees, then the incipient species will begin to interbreed because there is now no reason for them to not mate, because the previous prezygotic barrier is now abolished. They will no longer remain separated, they will over time either grow into one species as they continue to interbreed, or another barrier will be formed and reinforced by the apple maggots not having a preference and the hawthorne prefereing hawthorne. This allows for the separtion back into the two trees they prefer.
Part B: The two species will definitely diverge as they have a reproductive post-zygotic barrier that is causing hybrid offspring to be sterile. This will not allow the two species to merge in the long run. This is because the hybrid offspring cannot reproduce leading to a very short lived new hybrid species that is merged from the two. The two species with the mutations listed will diverge as they are unable to produce a viable merged hybrid off spring. If new mutations occur it could either reinforce this seperation or it could lead to a merging over a long period of time.