Okay... that's why I suggested you edit or remove the letter, not redo Cyrodiil to make it conform. We want consistency within the mod.
I also don't think TES6 will negate all or most of Beyond Skyrim (how much would an identical project based off of Morrowind have been negated by Oblivion or Skyrim? Did they really provide records of what was going on outside the previous games' borders?). It could easily remain headcanon for Elder Scrolls fans even after the new game comes out. Heck, in a universe where inconsistencies in canon is literally part of the canon, it could be seen as legitimate even if included the Argonians conquering all of Tamriel and founding an Empire of the Lizard People, much less ignoring the contents of an obscure letter detailing some military movements.