Why couldn't Jack return home?
So he loses the injection that will send him home. So what? They can send stuff back, they sent him a couple of boxes including the original injector, and an extra long-minute watch. When he doens't return they would probably send McNulty back to see what's wrong, he would report that Jack lost the injector, and they'd send him a new one. Instead he's just stuck forever? Worse that in the actual Trancers II they decided to bring him home with a riduclously complicated timeline wrecking plan.