The Wedding was staged.
Bob and the family used Leo's overprotectiveness of his sister to snap him out of his catatonia. Bob isn't as stupid as he seems.
Bob's insanity is an act; he's simply doing this to fuck with Dr. Marvin while at the same time bringing happiness to the people around him. I elect this as a theory since I think it makes the movie even funnier. Leo never tried to kill Bob.
He merely snapped after the incident at his surprise party and was temporarily disconnected from reality. The scenes with Leo strapping Bob to twenty pounds of black powder is just his own internal manifestation of the raging conflict between Bob and his own deep-seated personal issues. Their vacation home was actually destroyed by the Gutmans, who finally seized their chance to take their revenge and set the house on fire, blackmailing the Marvins to pay them the insurance money so that they wouldn't tell the press that the renowned Dr. Leo Marvin was seen assaulting one of his patients. The scenes that pick up with Leo catatonic in the mental institution are when the movie resumes. Dr. Marvin completely misses the source of Bob’s problems
In their first interview, Dr. Marvin is so conceited and convinced his book can solve Bob’s problems, he completely misses the source: Bob’s divorce. Bob has a preoccupation with family ties, which probably stems from his wife leaving him. Dr. Marvin assumes his problems began before his wife left him, but we never see any real evidence that’s the case. By gaining a de facto family over the course of the movie, and finally marrying Lily, he’s finally able to overcome his problems.