I don't think that Tempus needs to have put kryptonite in the other dimension with Kara. He's not there to make sure it hurts her or Clark, after all. All he needs to do is destroy Herb's beacon so they can't return.

Given that Herb was expecting Lois and Clark to return only a half an hour later, why on earth did he feel the need to answer the door? He doesn't need a back story or excuse for why he's at their house. He just needs to stay in the secret closet until they return.

I'm not sure how good a shot Herb is, but I suspect that he wouldn't be that accurate. His position is really compromised by Tempus's belief that he won't actually shoot him. Maybe he could shoot Tempus in the leg or something non-vital. I guess that could still kill him by bleeding out, though.

"It is a remarkable dichotomy. In many ways, Clark is the most human of us all. Then...he shoots fire from the skies, and it is difficult not to think of him as a god. And how fortunate we all are that it does not occur to him." -Batman (in Superman/Batman #3 by Jeph Loeb)