If he was so left handed that he was unable to learn to write properly (right handed) he should have had similar problems learning to use weapons properly (right handed).
But if he was ambidextrous, he could have had learned to write with his right hand.
I don't think Sokaku's alleged illiteracy was caused by the attempts to correct his left handedness. I'm more into a disability...
The Connection Between Dysgraphia and ADHD
Dysgraphia and ADHD may seem to be closely related, especially because most students with ADHD have other comorbid disabilities (or disabilities that exist along with ADHD). Dysgraphia is often among these comorbid disabilities. Researchers have wondered whether dysgraphia and ADHD are causally related, in that one causes the other, and if so, which causes which
A 2007 study at Bar Ilan University in Israel found that ADHD students were more likely than their peers to have difficult in writing, even if they had normal reading skills. They found that ADHD students were far more likely to omit, reverse, or add letters to words (mistakes known as graphemic buffer errors), exert abnormally high levels of pen pressure, and write quickly and inefficiently. They concluded that these errors were due less more to non-linguistic problems (although linguistic problems did play a part in some cases). So according to this study, dysgraphia and ADHD are somewhat causally related, in that ADHD can lead to a dysgraphia diagnosis due to the student's intensity and inability to slow down as easily as her peers.
On the fingernails burning... Who said it was for his resistance to learn to write?
He (Sokaku) next talked about how quickly Marquis Tsugumichi Saigo learned techniques and having met and talked with General Nogi in Nasuno (in Tochigi Prefecture) and how much he liked it when the latter dressed up like a farmer. He also told me that since his martial art style was very easy to learn, he had never demonstrated in front of people. Takeda Sensei said that his father cauterized the nails of both his hands everyday as punishment for his not being able to learn techniques fast enough. He showed me the burn marks still remaining after so many years. Even when the hour reached two o'clock in the morning he continued to talk endlessly.
Who said Sokaku couldn't read?