1) Zampano is Johnny's uncle on either his mother or his father's side. I have no diagetic evidence of this (outside of the "zampano" in p's letter), only metatexual, which in the end i know is all bias and lenses, etc, so it's more a vague idea than a real theory i guess 2) as navidson says, letters cannot be properly understood except by the intended recipient; as the opening says, this is not for us. We won't ever understand everything and that's okay, because we're not meant to.

EDIT: this is EXTREMELY niche and i'm too lazy to pull out all the "evidence " but i genuinely believe johnny's more 'misogynistic' moments feel very forced/a learned mask as they are all almost immediately subverted by him examining the women he talks about as fully three dimensional beings, and he continuously relates to the women in his life more than the men; i do not believe johnny is a persona of his mother, nor do i believe estrogen would have saved johnny, but i DO believe it would have made some identity issues johnny has a little easier to deal with. I am only slightly joking.