If you damage the spring assemblies that control head actuation, you are currently stuffed. Once you bend the spring beyond UTS, a drive is broken. You can do this by forcing a disk from a drive or plain meddling.

"2) Sony MP-F75W-11G - Disks do not clamp down correctly when inserted": this suggests a problem with the cam/lever mechanism. Dig out a working drive and observe how the mechanism works. Compare with the faulty drive. I suspect that you are missing a spring or that one of the levers is bent.