I prepare a lamb stew braising lamb shoulder but have lamb leg in freezer. Does lamb leg braise well? Enough fat and collagen?

Sure you can do it but...A 50.00 lamb leg as opposed to a 20.00 slab of stewing lamb or shoulder. Save your leg for a special occasion and get yourself the cheap cut.

Seven-hour leg of lamb is a French classic. Those recipes (and there are lots of them!) don't call for de-boning.

Yes you can, but I would de-bone it and butterfly it so it submerges better in the braise.

Legs have a lot of connective tissue, so that's where the collagen comes from. You don't need a ton of fat since most of the flavor comes from your sear and your flavorful cooking liquid. A leg should braise beautifully.