I also would not re-finish to fix a few scratches. I don't think the problem is oxidation related. Wood cannot oxidize under a finish, no access to oxygen. Could be UV related, but Cedar is not known to be highly photoreactive. A Lot of the color shift that happens over time is in the finish (on most conifers anyway). Nitro ambers a good bit with time. Most other finishes do to some degree as well. It looks like the surface was not sanded all the way back to white. Did you check it with a wipe of spirits before sealing? spots should have shown up then. When shellac is used on bare wood it actually penetrates into the wood a little bit and grabs onto the fibers. Locks in so strong it will seal in anything, sap from knots, smoke stains, ink, anything. That's why people like it as a base coat. It can be real difficult to remove. Lacquers and a lot of other finishes on the other hand just lay on the surface, requiring a rougher surface to grab onto. A vinyl sealer instead of shellac may have helped by bridging the bare wood better, but I think it would have still been noticeable.
So what to do now? I would take the wood back entirely to white. Starting with a good chemical stripper. Be careful, or you will wind up re-finishing the whole guitar. Masking tape is no insurance when working with chemical strippers. The tape is on top of the existing finish and the stripper will eat it right out from under the tape. Small pools of stripper and a card scraper works nicely. Maybe some maroon or grey scotchbrite soaked with stripper would help. Make sure to test your stripper to be sure it does not eat scotchbrite before using it, that would cause a disaster. Don't use steel wool. Work small areas at a time, very carefully until the top is done. Sand the top lightly and check with a wipe of alcohol or naphtha. Any spots that show when wet will need more attention. Depending how much French polishing you did and how hard the shellac was packed into the wood, you may not be able to get back to white without getting to aggressive on the soft Cedar. In that case you may need to follow your original process again to eliminate the problem entirely.
Hope this helps.
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