Jor-El would be in on the secret because he's part of the Council. But, I think it could work if they laugh off Zaar's thetas as the ravings of a jealous fool, and Jor-El's warning as trying to incite panic over the arrow slings of a relative caveman. I mean, who would expect one guy to be able to blow up a whole planet, right? And if Krypton is aware of the Circle of universe elders, then they know Zaar legally can't touch them because they haven't legally done anything wrong.
In such a context I don't think it's unreasonable to think they'd be pretty chill about the whole thing.