I think a lot of it is Chris backpedaling on things he's been told in letters, trying to get ahead of the fact that he was shipped off to the nut house. "Oh no, they only requested my attendance to teach me legal stuff!" sounds a lot better, to him, about why he went there, then questions of his sanity. What's also strange, is you'd think with his ego, that'd claim something like they shipped off there, but he emerged triumphant and told those nurses to go to hell. Instead he keeps tiptoeing around it and trying to downplay it. Getting sent to a mental hospital seems to be one of his continuing fears. Normally Chris doesn't care about stuff that scares him after awhile, but this fear is a pretty common theme.
I imagine it was finally drilled into his head that he wouldn't be able to go back home or be around Barb if he was found guilty, which up until the hospital visit, Chris didn't give two shits about. Now that he's actually gotten consequences, and huge consequences at that, he's actively trying to look for a not-guilty plea or something that will get him off the hook on this charge.
I wonder too if that's the case, or they told him there might be additional charges. If Chris was only ruled competent to stand trial in the last month or so, they may have waited for additional charges, especially if Heilberg was pushing for a competency evaluation at an official hospital.