I would just buy a new pickguard and have it glued on...
He's removed the finish. Spruce is fragile and porous. Finish slows humidity changes and protects the wood from staining.
It's not hard if one is patient.
I don't think he will ever match the finish unless he does the whole top (which I would not do). If I were to go that route, I would hire someone like you with the know-how. I have acoustics with and without pickguards. I can go either way...just seems like replacing the pickguard would be an easier solution even if the finish was removed where the old pickguard once was.
I'm reading the OP as he has stripped the entire top of finish. Perhaps he will correct me if I am wrong.
I didn't read it that he had only removed the finish under the pickguard, especially since he mentions sanding and sanding is difficult to isolate.
We are both thinking different things and neither of us is wrong at this point. So it goes... :- D
While I have experience in doing finishes, learned long ago, things have changed. If there is finish on the top still, I would not take it on even if I still had a place to do finishes (I don't).
Many newer finishes are UV cured or a new type of plastic compound. Lacquer may not be friendly to those types of finishes. The crackle finishes that we see on some guitars are based on a chemical incompatibility.
I sprayed lacquer over varnish once and it was a disaster, created my own crackle finish except not pretty at all!!!
Fortunately it was my own possesion so I didn't have to make good with a customer. I never experiemented like that again!!!!
French Polish is probably the OP's best solution. It's easy, you can do it in any well ventilated, reasonably warm area. Gloves are mandatory. It is not difficult and it is very hard to make a mistake since you can just rewipe with more alcohol on the rag and clean it right up. Cheers, Koru