Welp, I found one shriveled oto cat on the carpet the other day. It looks like these things can jump out of an uncovered tank. I've never heard oto's jumping out of tanks before, but now I learned.
Don't blame yourself - you did all you could - counseling, medication, electroshock therapy. He's in a better place now - a place where tanks have no openings whatsover. Remember him as he was in the tank - not the horrific, piece of fish jerky as you found him - it could have been worse - it could have been the apistogramma.
hmm I suspect aggression caused him to jump out. I have 3 otto in an open tank with whiteclouds, betta one platy and 2 cory cats.
The ottos mainly stay at the bottom or graze the plants. While the whiteclouds will leap a little when spooked, all remain in the tank (water level a few inches lower than the top).