How to handle a corner when using an L schluter edge?
A clean miter cut does indeed leave a very sharp corner. Since these are mill-finish, you can very lightly file or sand the resulting edge, just so it's not going to cut skin. You don't want to take so much that it's visually rounded. 

If that's not acceptable, the only other idea I have is to bevel the corners of your backsplash slightly using two 22-1/2* angles, leaving a small flat where the original corner was. This does result in a very different look, though... almost a ranch-style effect.

Update: If you were to cut away the entire base flange for a short distance, you could probably bend the remaining face into a round corner. You'd have to experiment a bit to get something that doesn't tear. Aluminum seems soft, but it doesn't handle sharp bends like steel does.