Thank you for the great detail Susan!
What rattios do you use when mixing up a batch of shellac?
I always use scrapers on old finishes first. A couple of responses said never use scrapers. I'm not sure why not. A sharp scraper will remove the old finishes quickly and easily without removing wood. Learn how to sharpen a scraper if you aren't sure. Near the bridge a sharp chisel used as a scraper works very well. Just work slowly and carefully so you are removing the finish and not the wood. With a sharp scraper a relatively light touch is all that's required. At the top of the bridge where there's little room to work a chisel is about the only tool that gets you in there.
Wipe down with denatured alcohol to remove any residue as then sand lightly with 200 grit and then 320 before sealing with amber colored shellac (dewaxed). Sand smooth after sealing with 200 grit then 400. Light pressure. I usually use slow drying, high gloss, spar varnish as the final coats (2) sanding lightly with 320 or 400 between coats. Two coats is enough.
Sometimes the old boards are not so even in color, or aren't well grain marched, and many of the old boards seemed to use shellac or varnish with some kind of darker stain to make the panel look more uniform. Some sections where the grain was more open seemed to take these stains more and can be difficult to get out, I don't.
I don't like to bleach boards or aggressively sand them which may thin a panel that is already a little weak. However if you are using an epoxy treatment you probably have more leeway there. I don't sweat trying to make old boards look perfectly uniform in color.