Bob, you may be fighting a losing battle. The leather in these old cases was vegetable tanned and time has probably deteoriated it significantly. And as you've noted the flax thread has also rotted. Leg of Mutton cases are very difficult to repair properly unless you're only talking about hinges, end caps, etc. even something that appears as simple as replacing the hinges on the handle are very labor intensive due to its location--hard to work a needle halfway down the inside of the case. I have a working knowledge of leather (grew up on a ranch when we still used horse and harness) and have rebuilt/repaired a number of mine and if I'd charged myself minimum wages the cost of the repairs would have bought half dozen decent cases. I think working on boxlike cases would be simpler in many instances. Sorry to be the bearer of bad tidings but suggest you cut your loses now.