a letter is indeed a primary source. if that letter exist and is authentic. yes it is a primary source.
a memory of what is written in a letter is not a primary source.
between bias, failed memory and paraphrasing the content of a letter.... none of those are considered primary sources
Often , you can get assistance from researchers who may be able to do your research for a fee,or help at least locate sources. They could do this for you prior to your arrival and make research go deeper or more smothly. College kids would love to do thisand you might benifit fro it because they know the system and the ins and outs of research. Maybe contact them prior to your arrival and yo could cover enourmous amounts of ground to vast depths in the resolutionof this issue. Either way It would be interesting no matter the outcome. Are you writting a book? I'd be very interested in that as well whenit comes out?
Kinda fun to find out the facts what ever the outcome. I am as concerned as anybody that the grey area's in this matter are bulked out with unsubstantiated speculation. It seems harmfull to control information to the point of distorting it simple to weild power as the pupet master. research is always more interesting and fun.