Dan Koretz wrote:

One reason is that with a feathered
paddle, it is nearly impossible to use the power face because to do so,
you would have to bend your control wrist to an extraordinary angle.

Actually, it has nothing to do with feather. What you say applies equally well
to unfeathered paddles. If your elbow is below the paddle, the power face
points down. If above, the power face is up. Hence, in the first case you use
the power face, in the second, you use the back face.

The second reason is that it is hard to scull with the concave paddle
face facing down. The blade bites and submerges.

It sounds like you need to practice sculling. It is equally easy with both
faces of the blade. If the blade is diving, you're not controlling the paddle
properly. Don't use a "control hand" - use the hand closest to the blade to
control the blade. Your "control hand" applies only to forward paddling.

The answer to the second question is much simpler: the more your arm is
extended, the more leverage there is on your shoulder, and the more
likely you are to screw it up.

One forgotten way to minimize these problems is paddle extension. Don't reach
with your arm - slide the paddle blade out and keep the arm in.

Mike