Nitro is a solvent release finish, like shellac. You mix the stuff up with the proper solvent and paint/spray/pad it onto the surface. The solvent evaporates and leaves the finish behind. It doesn't 'cure' in the sense that a varnish finish does, by a chemical reaction that changes the nature of the finish. Lacquer thinner is much more volatile than the alcohol that is used with shellac, so lacquer hardens faster. It can still take a few weeks or longer for all of the thinner to be released, but shellac is still slower.
BTW, I'm told that the least toxic thing in lacquer thinner is toluene. That's toxic at a concentration 1/10 of what you can smell. Wear a hazardous vapor mask with a nice fresh cartridge when you apply it. By comparison, if you use pure ethyl alcohol to dissolve shellac the thinner is non-toxic in moderate amounts. Shellac itself is, at worst, inert: you'd get a tummy ache if you ate a lot, but it's not poisonous. They do use it in food as a coating in small amounts, I'm told. The other thing that's used in French polish is oil, often olive oil.
Any finish primarily changes the tone of the guitar according to how much of it there is, and how stiff it is as a material. A very thin coat of almost any finish will have pretty much the same, minimal effect on the sound. One reason FP is so good is that it goes on thinner than most other finishes. Of course, a really thin coating of even a very hard material won't protect the top against dings and/or scratches, particularly if it's a soft wood like cedar. I've been using a very hard and tough oil-resin varnish of late that can be applied only a little bit thicker than FP. It gives more protection, without a noticeable penalty in sound, but it can still be dented easily just because it's so thin. We're talking about a thickness of about .002" (.05mm) for FP, and (with luck) .003" for this varnish. Manufacturers can put on sprayed lacquer finishes at around .005" if they really try to keep it thin; .007" and more is more common. The prize (?) goes to an Ovation guitar I re-topped a few decades back that had .04" (a full millimeter) of epoxy on the top for a finish. You could not scratch it, but the top could not move much to produce sound either. That's the trade-off.