Hi, I’ve been lurking and reading all this for some days now but handwriting analysis is my jam so I had to register to just say this:
If you look at the writing on the envelope addressed to Addison and then compare it to areas in Addison’s own diary (places where she writes in all-caps like the section where she talks about moving in with Sadie) the letters look extremely similar to me, specifically the internal loops on the double Os and swirl to the right at the end stroke on the Ns. It makes no sense that Addison would be writing notes to herself but I can’t shake my suspicion that they were probably written by the same (out of game) person.