Just a question to the question you posed NX...why would John even tell Sarah what he did? Unless it became absolutely necessary for him to explain his actions I think he'd remain silent about his actions regarding his father. Perhaps if Sarah presses the issue with him John would reply "Everything happened like it was supposed to Mom and that's all you need to know." I think that'd be an extra burden for John to carry around that he doesn't need if he were to tell Sarah about what he did in the future...he doesn't need to alienate her. Besides which he has Allison to confide in about the future events right?
Unfortunately I don't have time to expand upon it, but I would've expanded upon it as the story progressed (this was going to be pretty much the first act in a multiple arc story I'd been thinking up). Why John would eventually confide in Sarah with what had happened and his choices to send the terminator back to kill his own father in the end would've been revealed.