What if the assessment comes back supporting everything this psychiatrist has said?
Then that will be taken into account by the court.
It's not solely about finding him guilty - it's about delivering a verdict in possession of all the facts relevant to the case.
There's also the secondary consideration; if he lacks the mental capacity to act in accordance to his understanding of right and wrong, then that raises the possibility that under similar level of stress, he may not be a reliable witness to the events that happened.
I think if the Defence thought that all that would happen is that another psychiatrist would agree with the diagnosis, then they wouldn't be too worried about it happening, other than the fact that it would cause an unnecessary delay.
The thing is, the risk to them, is that for an amount of time, he will be under intense scrutiny, and everything he says or does will be reported back to the Prosecution. That may be a huge risk for them. It may not, but it may.