I had an ambi selector on my AR for a short time, and didn't like it. As I rotated it down with my left thumb, the lever on the other side would dig into my trigger finger.
I was speaking with a lefty three-gunner (I shot 3-gun too, but it was 6-8 years ago), to see if there were any new techniques for lefties shooting "right-handed" ARs, and he handles the gun the same way I do. Trigger finger on the selector and bolt release, right thumb on the mag release (works especially well if you intend to retain the mag).
I've never had an issue with right-side ejection.