He can't prove "legal" insanity. To be proved incompetent to stand trial, its not a matter of being crazy, its proving if at the time of the act the individual was able to distinguish right from wrong. The fact he hid the body, and is a repeat offender alone point to the fact he is and was aware of right and wrong. Legal insanity isn't the same thing as psychologically insane. Its only judging the competency of the individual and his awareness to what is going on.
Camblos is no dummy he knows he can't try this case on the grounds of insanity. But he may want to use it at the time of sentencing...to perhaps get JM in a facility rather then a state prison.
Anyway, OceanBlueEyes did an AMAZING job explaining all this in the thread regarding when charges will come, you should read it. Its awesome!