I think you're technically correct - there's an edge case where you extend time to allow the PK and the resulting kick spins back into the goal as per the video posted earlier. Unlike KFTPM, a defender would have to be allowed to come in and clear the ball (or even just control it to end the game) and you're right in saying that you shouldn't discourage that from happening.
Having said that, I think the benefits of explaining the situation clearly and stopping any post-match arguments before they begin does outweigh the tiniest possibility of something mad like that happening. I still think it's prudent to explain that it's going to be the last kick of the game - how the players take that and what they do with that information is up to them.