I think you did the right thing in getting a spare stock to use at least for the time being. Then you can take your time over the original stock. Read up all you can before doing anything irreversible.
There is no doubt that the present state of the surface can hardly be described as a "finish". It will all have to come off. I would use the jelly-like paint stripper that feels cool and then burns on your skin (is that the methyl-hydrate recommended by Sentryduty) but try and avoid using ANY abrasive.
Methylated spirits will also dissolve shellac, so you might be able to remove the present wrecked finish with meths alone. But the stock will also have waxy/greasy material in places, so the best bet is stripper followed by meths.
For refinishing I suggest one of 3 options.
a) Simply oil the stock inside and out with linseed oil. My preference. After all, Russian
rifles also used oiled stocks, and I suspect that an awful lot of shellacked stocks are post-war reworks. And oiling does not prevent you applying a different finish later, should you so wish.
b) Simulate the original shellac finish (if it really was original), according to the recipes which you will be able to find elsewhere.
c) Although I have never used it myself, it seems that Tru-oil would provide a good finish, and be a lot easier to apply than a shellac finish.
But I would NOT use spar varnish. Too gooey and shiny. Messy to remove if you change your mind later.
Whatever you do, avoid soaking the stock with water at any stage. Or say goodbye to all the little stamps that you can probably find in various places on the stock.