I'm one who highlighted the favored phrase of Burkett - "run interference" - coming out of Killian's mouth in the CYA memo. And Michael Dobbs of the WaPo found THAT AND other dead ringers.
Burkett is the forger.
Unless the forger framed Burkett in case the forgeries were caught, by using Burkett's published lingo to hide his/her own guilt. Then gave Burkett the memos, & he ran with them.
What else is feasible?
Good job! Certainly, there are forensic people that specialize in writing style, etc. It would be interesting to have them confirm your observations. Unfortunately, that will only happen if this is investigated as a crime and I don't think that will happen any time soon.