An article from the Smithsonian museum suggests a more complex history , and describes the "avoiding common letter pairs" as debunked by the fact that ER is a very common pairing. It suggests that perhaps the profitable training courses helped lock the users to particular manufacturers, and also describes some 2011 research which suggests that Morse code transcribers, an important early set of users, drove some of the design.
AnThe history of typewriters is well studied by industrial historians. For a trail into that world, this academic journal article covers the literature Kroemer, K. UAIS (2001) 1: 99. "Keyboards and keying An annotated bibliography of the literature from 1878 to 1999" https://doi.org/10.1007/s102090100012