Writing was taught very differently in the Victorian era which makes it harder for us to understand and be able to transcribe accurately, but I think the simple question is this, whether or not Jack wrote the letter (and I personally believe he did):
Is it more likely that the killer was clever enough to deliberately lead the police off in a false direction by trying to imitate an accent in his writing; or more likely that the writer was a semi-educated individual who was writing as best he knew how?
An interesting case from the Old Bailey 1882. Scroll down to the evidence of WM Walter Morton who mentions, " a tall dark man who appeared like a clergyman". Further down, the evidence of Garratt Redman mentions a Rev Dr Flanaghan, Rector of Kilvenny, who apparently never existed.