I worked a lot on Healthcare at McK. Happy to help:
1) It's not limited to supply chain and projects may include anything, even improving directly the medical outcomes.
2) As far as I know - consultant is a track with the progression to the partner. Coach is not.
3) It was lower in implementation 3-4 years ago. However recently they added the partner track (consultant role) with the salaries similar to the generalist Associates / Managers / Partners. Can't give any details re coach role.