Ah the old time paradoxes, good thing about the show (and maybe movies) is that people don't seem to change/fade away if they have come back in time and then are erased. "Our" Derek seemed to have been erased from time and was not the same as Jessie's Derek so in theory John could get back to the past with Cameron's chip, stop skynet and still fix then keep Cameron, you know just in case.

Yeah I've not thought about this way too much at all.