I have a hand cut slim-jim holster (likely CSA) that was in worse shape than this pouch. It was found in a demolition of a pre-Civil War period structure in Baltimore. The holster was crushed flat, not flexible at all and covered with mold as a result of use, poor storage and abuse. I gently removed the mold with a soft brush, soaked it in room temperature distilled water until soft and pliable, opened it gently, slipped my original Colt Navy into a plastic bag and carefully placed the gun into the holster. The original leather stretch/wear areas matched those of the cylinder and muzzle of the pistol perfectly. After allowing to dry slowly I removed the gun. The holster was stiff again as expected. Applied minimal leather treatment to soften. No hard spots, flexible. Success! BTW, I try to check my leather yearly to remove mold or verdigris as needed. I'd give it a try on the pouch. Suggest using bubble wrap only to expand and dry, etc. Good luck.
Hopefully, I can post some photos when I dig out a bit here in Colorado.