green screen shakes and shivers

When you slow down footage you are creating frames where there were none before. I'd suggest you do this in a two step process:

1. Slow down your footage and render it. export selfcontained movie, do not recompress all frames.

2. re-import the slowed down footage and apply green screen filter.

also, sending the clip with the slowdown applied to compressor and changing the blend mode to optical flow usually noticeably improves the quality of the speed change. Not sure if this would have any impact on the quality of the key. Probably a good idea to do a test.

I've had good luck down to 20-30% speed using Shake speed change in adaptive mode. The same basic capability is available in Motion and Compressor but it's called Optical Flow. It actually predicts and generates intermediate frames rather just duplicating.