I disagree strongly with the notion that Cai punishing us harshly makes her 'lawful stupid.' It makes her lawful good.
We committed a crime while being a inner circle council member. We violated the terms of her peace and unlike Ji Rong, actually committed tangible harm to people- harm that won't be fixed by a payment of a few hundred spirit stones. And we were planning on corrupting her very justice apparatus in order to harm a third.
Now you might say that it's better because unlike Ji Rong, we would have confessed and apologized. I'll grant that. But on the other hand, Ji Rong's theft was callous and casual- he probably was acting like all the corrupt cops/guardsmen he encountered in his life and didn't give the petty theft a second thought. Our crime, however, was completely and utterly premeditated.
There's no moral nuance here, either, like some have implied. We didn't steal bread to feed a starving holocaust orphan. We didn't smuggle food to rescue an army. Our motives were selfish.
The only thing going in favor of leniency is our close relationship with Bai, assistance her war, and position in her inner circle. But that almost makes the crime worse by making it a betrayal. And leniency on grounds of personal relationship is exactly the type of corruption and nepotism that Cai says she's trying to to root out.
Now, I don't actually expect her to lock us away for a month, if only because I don't expect the GM would have gone that far, but basically any other punishment is on the cards. I would have expected at the very least loss of all our inner circle benefits- White Room, pull furnace income(she may not confiscate it, she can easily stop giving us generous terms), tax exemptions for us and our friends. That's not to mention all the other relationships this would damage if it became public.