when the varnish/the color on your guitar is too thick, the body of your guitar will loose most of his resonance. my guitar dealer painted one of his 50€ strats and when he was ready he played a few chords and threw the guitar in the trash
2. if you want to do it - don't do it for yourself and be sure that you remove the old color of your guitar. the thicker the finish is on your guitar, the worse it is for the sound.
I don't think so but you can ask them. No, I think that no guitar manufaturer will make a new finish. They will tell you that it's not a good idead and it's also really expensive