This sounds like an interesting project, if you can pull it off. The major problem I see is finding a reversible adhesive that could hold your fabricated leather pad on securely, and without gaps, and also be easy to remove without damage in the future.
Titebond makes a reversible glue that I believe is a hide glue. Hide Glues can be softened and removed by applying water, steam, or heat. But that means your pad might become loose if it got wet in a duck blind or boat. Hide Glue isn't very strong either.
Silicone can be used as an adhesive, and it can be pulled or peeled off without too much effort. It is waterproof too. But personally, I don't like using any silicone products on anything that might need refinished at some point. Silicone contamination can give you fits during wood refinishing or rust bluing.
Auto Body shops are using Hot Melt glues to glue pulling tabs on dented body panels in the PDR (Paintless Dent Removal) System. After the dent is pulled, they use a solvent like alcohol or acetone to soften and remove the Hot Melt Adhesive. The question is whether you could get enough applied to your stock butt or pad, and secure it in place before it cooled, and would no longer stick properly.
Before I committed to doing this to a nice gun, I think I'd experiment with a junk stock to test adhesion, appearance, and ability to reverse it. Nothing ventured, nothing gained, but if it didn't work, it would be better to find out on a junk stock.
I've used a local Amish Tack & Harness Shop to do some repairs on old Leg O' Mutton gun cases. I was surprised to see that your Amish leather guy has a website. I never heard of any of the Dutchies around me using a computer, and there is still a pay phone a few miles up the road from me that they use to make calls. They plow with draft horses, and only use some old tractors for stationary power... but it can't have rubber tires. The Dutchman who made the oak flooring for my house had electric power running right past his house, and to a service pole behind his wood shop. He used electric power to run a single large motor outside, and a flat leather belt ran inside to turn an overhead line-shaft which ran his saws, jointer, planers, molder, etc. Yet even though he had power right out back, he used kerosene lamps and Coleman lanterns for lighting inside the shop. They are stubborn and resist change, so they must have been very upset about things to come out and openly support Trump this year.
