I don't see the point that the pro-Jeremy side are making here. He knew she was ill. He knew she could 'go crazy'. He knew she was violent. This knowledge does not make him guilty, but it was enough to form the kernel of a plan, if he is guilty.
Knowing she was a "nutter", that provided the backcloth to his staging plan for Sheila. For context, the reason this conversation started is that some pro-Jeremy people are trying to suggest he would have needed in-depth knowledge of Sheila's condition to do this.
On the other hand, he didn't live with Sheila and hadn't throughout the relevant period of time, so there was no reason for him to know much beyond the basics.