dd,

I have a Mohawk double paddle they advertised as being for canoes, at 9 ft, it allows you to keep your stroke " flat" even in a tandem paddled solo. You have a true solo, so your extension should let you flatten your stroke and save your shoulder.
I have the click lock, set at 0 and 60 degrees ( I believe that's correct, never really checked)
I tried both positions and found that that for me, the 0 degrees was easier on the wrist/forearms and seemed more suited to the flatter " touring" stroke. From what I could tell, the angled feather setting was more suited to a higher angle/cadence windmill type stroke.
Kayak guys might see it different but that's what I found.

I think if you have rotator cuff issues the 0 degree setting or maybe a compromise between the 0-60/70 degree setting might be the better choice. If you can get on the water locally, I'm sure your shoulder will tell you what feels the best ;)

PS, I believe Mohawk sells an inexpensive T-handle kit to make single paddles out of their double ones.