In my view, spelling and/or grammatical imprecisions are an irrelevant consideration when evaluating personal eyewitness accounts (e.g. letters) of historical events. That's provided the meanings can be deciphered. If anything, spelling or grammar mistakes seen in written letters add authenticity and believability to the contents read.
One must judge such materials according to the standards of that time, not of today, in terms of rules of spelling or grammatical construction.
I agree, there are cultural factors that can be quite baffling. Recently, the daily journal of a teenage girl living at the plantation at the end of my street was discovered. With great anticipation I opened the envelope containing my copy.
Page after page of beautifully written weather observations, quotes of CSA patriotic poems from the newspaper & census data on who came to visit or those visited filled day after day… ho-hum, I have seen this kind of thing before.
Every day young ladies were required to write in their journals. Every day, they presented them to their mothers for inspection. Understandably, the hopes, dreams, fears & joys that can make journals so fascinating are completely missing.
Perhaps that is why nobody reads George Washington's diary. Apart from historical meteorological studies, there is nothing to be learned.
I have a copy of Signal Corps Captain McClintock's meticulous daily entry journal. He stood next to Sherman during the Battle of Altona, the siege of Atlanta & was atop the cotton press when Sherman made contact with the USS Daisy, marched through the Carolinas & the victory parade.
McClintock barely missed a day writing in his journal. An example of the typical entry:
"General Sherman announced that Atlanta has fallen today."
Really gripping isn't it? Fortunately, he brought home a trove of original documents as well as the journals. His great granddaughter contributed them to the Huntington Library in CA. The archive includes the signal log of Kennesaw Mountain Station during the Battle of Altona & Sherman's handwritten signal announcing Lee's surrender. It is available online from the Huntington Library.
The lesson being that memoirs, journals, letters & diaries are only meaningful within the context of archives that surround them. There are objective facts & there are human experiences… the two often overlap in the most surprising ways.