While Mod Podge is a great product, there is one thing I just hate about it; it stays "tacky". Not in the tacky sense of things like with glue but in the sense of soft. That's why I prefer acrylic. It's a bit more expensive but good quality acrylic varnish dries hard. I tend to like the Golden, Tri-Art and Liquitex brands.
When applied with a foam brush, it doesn't leave any streaks. And unlike an acrylic sheet, you can get it in a variety of textures. But it won't be as resistant as a 1/8 sheet of acrylic.
BTW, if you want to go the sheet acrylic way, Ikea makes some cheap frames that have an acrylic front. Around here they sell for cheaper than the sheet of acrylic at the hardware store.
the streaks are true. I did use the mod podge on my bass cigar box and see the streaks, fine ones, but still streaks. I have not noticed the softness, I will try to test to see how soft it may still be after 2 days since I put the mod podge on.
My experience with just spraying over loose labeling is that it will still be loose when you're done.
I do use spray lacquer on my gits, but first I take the time to very carefully glue down every last bit of loose labeling. I use Elmer's Extra Strength Glue Stick. It sticks and holds virtually instantly. On very small bits of loose labeling, I dab a little bit of glue from the stick with a small diameter artist's paint brush and slide it up under the loose label being careful not to crack or break off the old brittle paper. (Yes, I do have a lot of free time to accomplish this task, and a lot of time it does take, but it yields great results).
I like lacquer because it dries in minutes, so I can recoat right away or get on to the next step in the build. Once I start a build, I like to go non-stop until it's complete.
That's an awesome box you have there and well worth the time to secure all of it's history.
Good luck with it! I can see that it will turn out to be a great guitar!
P.S. - I've never had any trouble with lacquer reacting with any graphics.