These are very, very interesting letters. The typed letter just stuns me. This guy is/was insane. Unless Waters translated/edited Brody's words, Brody is/was a native speaker. He was also a total lunatic, who sounds like he was trying very hard to manipulate Seka and her family.
The grammar and sentence structure indicate someone who was a native speaker; however, his word choice is odd. You said that he insisted that his dictation be written verbatim, so I guess that Waters probably wasn't editing Brody's speech. To me, it looks like Brody was a native speaker; however, due to his word choice, he was a native speaker who wasn't very well-educated during his school days. He reminds me of a little kid who has gotten his hands on a thesaurus and is trying to impress the teacher. The way in which he uses the word "repose" is interesting to me. It's not the way anyone, even a non-native speaker would use that word. Have you ever read the book "Confederacy of Dunces"? He reminds me of the main character, Ignatius J. Reilly. It looks to me like Brody either didn't receive much of a formal education, or he was a horrible student who later on wanted to make up for that lack of education by trying to do a lot of reading. There's something very insecure that comes across in his word choices. He's deliberately trying to make himself sound highly intelligent.
Why did Waters transcribe his work? Why didn't Brody write these letters himself? Was he able to write? Was this just part of his manipulation of Waters?
It is possible that he was raised by parents who were non-native speakers; but he was a native speaker.
This is just my opinion only. Do you have any other writing samples? I'd love to see if he's just trying to impress Seka's family by trying to make himself sound smart (and failing, IMO). I'd love to see if this is his usual writing style.