masking tape - removing it from walnut stock

[ssgross]

it's ruined as a collector piece

Many others share your opinion about removing the patina - and you certainly are correct about "most antiques". But people buy and collect antiques for many different reasons.
I spent quite a long while reading posts, likely some of yours, on the matter giving it due consideration. Two fortunate things considered 1) this krag, like all my rifles, is not a collector piece - very far from it. 2) Patinas can return, with time and use, and the way I did it will allow it to return. Certainly some may disagree as to the definition of "collector's piece". Perhaps that's what is so great about it, we each can create our own definition - analogously some would never consider regularly taking their "antique" rifles hunting, some think differently.
There are plenty of definitions of "patina" as well. Would you say that degreasing all the metal parts of old, inferior, lubricant and re-applying to better preserve the firearm is removing the patina - that patina has a different look and feel on the metal. Is sticky old cosmoline part of the patina? on wood or metal? On the wood, all these chemicals are known to attack and degrade the fibers - that soft mushy feeling on top of the wood could be considered patina as well. If the patina of the wood is it's color, character, and feel over time then I did no harm. It's all still there, which was the benefit I clearly stated here and elsewhere. If it includes all the oils, sweat, smoke residue, and other damaging chemicals accumulated with years of neglect, then yes, no apologies, I removed those.
To your other point...I don't think it needs to be reblued to be preserved for centuries to come. The barrel had already been refinished at some point - doubtful it was original considering where it came from. I may refinish the replacement magazine cutoff that is currently a shiny baby-blue. By the dictionary, one may call bluing a patina as well. Is a bowl of cheerios a soup? Is a hot dog a sandwich?
I very much appreciate your thoughts here and elsewhere. The OP now has many good options of varying degrees to consider - none of which include a dishwasher, bleach, or oven cleaner. I don't like goo-gone as it leaves a residue only removed with something else like mineral spirits or other. Likely spot treatment will always show something, as the tape embedded its own patina over time.