The CJ4 that you are mentioning was, I believe, flown by an owner-pilot who had just gotten his type rating and was maybe on his first or second flight by himself-- single pilot. Basically, he thought he had engaged the autopilot, but had only engaged the yaw damper. Then he let go of the controls and started fiddling with the FMS.
The PIC of the Jet Commander had almost 10,000 hours and about 700 in type. I don't know much about the copilot. So I doubt it was the same situation-- most likely I would think gyro failure, but an AC failure or spatial disorientation is also a possibility. Since this was before the advent of the CVR, we'll never know.
They had a radar hit at 5,000 feet. A light 1121 could easily climb to 5,000 feet in 90 seconds or less. If they attitude indicator failed and he rolled the wrong way, he could have done a split-s into the lake in 30 seconds or less.
The autopilots in those airplanes were fine for the era, but not great by today's standards. With an experienced crew, I would guess they were hand flying, although a failed gyro would also cause autopilot issues, and I don't recall what warning or disconnect features it had. I do know this was before the little standby gyros were required, so he would have only had left and right attitude indicators.
