Another point of interest to me as a piano tuner is tuning the tone bars to a specific frequency. We modern piano tuners almost always tune to a "tempered scale." As you probably know, this means that, except for "A440" the notes are very lightly flattened so as to be able to play the piano in any key. When I restring a piano, I check its crown and also tap it while listening for the sound of a rich kettle drum (since no strings are attached at that moment.) If I were to tune the braces to a certain pitch, I would be producing an instrument which would sound good in only one key or pitch instead of any key. So when I read about tuning the tone bars with my peterson strobe tuner, I found it interesting but not a sound musical concept. One should listen for a resonant vibration from the soundboard instead of a certain pitch so as to be able to play beautifully in any key.
As to finishing the interior of guitars, the reason why mostly the theory goes "if it hasn't been done in the past so why do it now" is because there is no DATA for finishing the interior of guitars. Few do it, virtually nobody did it, and so there is no convincing data to persuade makers to add yet another procedure to their lengthy list--given that very few of the great patriarchs of the craft ever did it, and if you did it there would be no way to clearly determine if that it was indeed protective in some significant way--or to assuage the fear that it negatively affected the guitar in some way.
Stuart Mossman was the only serious builder I know of who finished the interior of his guitars. But he's dead, gone and all but forgotten; his instruments were quite good, but not memorable or highly prized now. And the finished insides of his guitars looked peculiar, besides. In the absence of clear evidence or a track record established by someone else that to do this is somehow good, no established build will do it. But maybe a newcomer such as yourself will to try it, utilizing the logic from another trade.
But I guarantee you. If your guitar comes out with a disappointing sound, you'll be nagged by the idea that maybe the extra finish "deadened" it. If it comes out sounding great, you may wonder how much better it might have been if you hadn't finished the interior. Or you may not. You might be able to present it as evidence that it doesn't hurt the sound much to do so. But you couldn't persuade folks that if it never cracks, it was the extra finish that guaranteed it--and not that it would have never cracked anyway. A lot of guitars never crack.
2- You're asking the wrong guy about "tuning" braces. I've advocated for years that all this "tuning" stuff for guitars was delusional nonsense, if not outright hogwash. I came to that conclusion after fathoming from my apprenticeship with a guitar acoustician that the guitar is an fantastically complex vibrational system, far more complex than any of the bowed instruments, certainly far more complex than all these folks talking about soundboard tuning--appear to grasp.
When you see the tests, like I have, those that show incontrovertibly that, for example, the headstock is the most acoustically active part of the guitar at some frequencies, and see over what an amazingly wide spectrum of frequencies the soundboard is simply silent, it drives you to conclude that all this preoccupation of tuning a certain brace or another to one note or another is just not worth the trouble.
The guitar is essentially a series of linked oscillators, or springs. Maddeningly, they all feed back into each other recursively. The strings are springs. The saddle bone is springy. The bridge is springy, the soundboard is springy, the neck is springy, and when you pluck a string, it sets all the springs dancing. And then the jostling guitar feeds energy back into the strings altering their
Some of the confusion emerges, I suspect, from the fact that these cultural artifacts that motivate and compel us so, are called "instruments". It makes one think of an oscilloscope or a frequency generator, or some such precision scientific apparatus. The guitar is not some sort of energy-transforming device that someone invented that can be effectively tinkered with to make it more "effective" or to "optimize" it. Optimize it to achieve...what precisely? It is a cultural artifact, originally devised to satisfy the aesthetic preferences of a particular ethnic group at a particular time in history. It has been modified over the centuries to fit the evolving vagaries of culture, fashion and taste, not to somehow optimize it in some objective scientific/acoustic sense. There is no objective acoustic goal for tinkerers to reach. And if you find some kind an objective goal, you'll soon see that it, too, is a moving target.
So I've shed all that baggage, and have come to see the guitar not in terms of an acoustical problem, rather more as an architectural problem, an ergonomic problem..and an aesthetic problem. My experience is borne out daily that when these factors are well resolved and reconciled--something which is very achievable--well, the result is a product that satisfies, sometimes even enchants, skilled and discriminating players--and listeners besides.