General Hollister's idea that the witness saw him at two different times and "telescoped" the two times so it was one man shooting another sounds pretty unlikely. (And when he said that, he didn't even know who the witness was or that people like her mother considered her unreliable, or else it might have been a little more plausible.)

Columbo's barely disguised laughter while Alex Benedict describes the emotional turmoil of the artistic mind and why it could have been suicide is a wonderful scene. It's completely clear that Columbo doesn't believe a word of it.