Well shoot! We've got nothing better to do than to speculate, so I'll play! :cheer:
If this is the case, I don't think it has to do with giving Casey time to think about it overnight. IF she fired JB, this could be a huge detour for CM and the rest of the DT. I KNOW that Baez planned on giving the opening argument. He said so himself - promised us we'd have that "AHA!" moment within the first 3 minutes. No matter how well you know the case, you don't just draft one of those in a few hours or overnight. Those are usually a work in progress over weeks. CM would need some headspace and time to come up with something... And yes, I'm aware we are still a few days away from opening arguments, but if JB WAS fired, I can see HHJP feeling compassion for the rest of the DT to get ready in 3 days.
eta: I am NOT in any way suggesting it would delay the trial. Just that HHJP would have some empathy for the DT.