Greetings Naty1.
The four most common professional solid body guitar finishes are nitrocellulose, polyurethane, polyester(all waterproof),and acrylic enamel, the most important type of which is automotive enamel(also considered waterproof. All of these will however, break down in time if in constant contact with "the elements". With any of these finishes, properly mixed and applied, one could leave a guitar so finished out in the rain for 24 hours, and not increase the water content of the wood by more than a miniscule amount, if any. Even the oil based acrylic would probably be considered to be truly waterproof, except when in constant contact with water. They are made with modified, polymerized oils such as Tung, Linseed, and Soybean oils. Thus the terminology of long oil, short oil. This refers to the polymer molecule, synthetically produced from the oil. At least, I would say "oil and water don't mix".
I believe that you are correct in your feeling that wood gets stiffer with age, and that this is due to final drying of wood. I worked as a carpenter when young, and the company I worked for used kiln dried wood exclusively. It was still commonplace to cause moisture to"squirt" from the impact of a pneumatic nail gun at 120 p.s.i. I once cut a large Walnut blank from a tree that had blown over in a storm on my property. It measures 8" thick, by 13" wide,by 4 and 1/2 feet long. It took 7 years of indoor storage for it to dry out enough to make wood lathe turning bowl blanks that would not further dry, and warp. Now, when wood is truly dried, the commonly used finished for solid body guitars, if not absolutely waterproof, ARE water tight. Humidity changes could not affect tone enough to be noticeable.
Stiffness would do as you said and affect string resonance. The less prone a wooden neck is to deformation, the more sustain a plucked string will have, and this effect will be more pronounced with heavy strings/lower fq, than with lighter strings/higher fq. A stiffer neck will not only sustain longer, but it will give an overall brighter tone. Still, I believe that all of this(stiffness of neck,mass, design symmetry, finish, etc.) does not add up to anywhere near the tone shaping capabilities of the electronic circuitry. In fact, I guarantee that for no more than $5.00, I can make any electric solid body sound brighter/more "treble", or darker/more "bass"). This would be the cost of changing guitars tone cap(s), or in the extreme case, using a small inductor instead.
Therefore, my main argument is that all the big "to do" about"tone woods"(which are very expensive), which finish to use, how stiff a neck should be, completely PALE when compared what one can do electronically to get the tone that they desire. I am only talking about passive electronics...caps,inductors,resistors,pots,pick ups, and modifying such circuitry that exists, from the factory, so to speak. Anyone that wishes to spend $300.00 dollars for a KOA body(that's from a guitar parts dealer like Allparts.com) can do so. I will save at least half of that to get the "Koa tone", and even more variation, by tweaking the electronics. I've been doing just that for the better part of 33 years of playing, repairing, modifying, and finishing guitars.
Don't mean to get up on a soap box, However, I have just gotten tired of the hype by people who SELL "tone woods", or say that only maple necks are any good, or try to sell one on getting, or doing a nitro finish, when it yellows with age, can be damaged even by alcohol, and is highly toxic to work with, and on, and on, ad infinitum. What made the electric guitar great ? Electronics ! Electronics is a field that is based on pure science, and theories proven. Show me the thousands of pages of scientific literature that have been produced, to show that one species of wood is always going to sound in any manner, when compared to another species. Wood differs greatly within one tree, even one board. One could say" scientifically", that Swamp Ash has many different tones, but even so, there exists only a fraction of a percent of scientific fact to back that statement up, when compared to something about electronics like :" a .047 tone cap can give you a darker/"bassier" sound than a .022 cap can". That quote is a scientific fact, all other factors being the same.
Thanks again for your input and time to do so, it's all valued by me, and makes one constantly re-think their position.This prevent stale thinking ! Best Regards...tonequester.