"Does Higgins seem to you to be an internally consistent, free from hypocrisy, self-possessed fellow? Is there not more than a hint of suggestion that he may hold others to a higher standard than he holds himself? Are we not all guilty of the same to greater or lesser degrees? Is this not part of the appeal of this and many other shows, to provide a mirror to our own humanity, grammatical errors and all?"
I think your interpretation is 100 percent valid, even though I don't personally agree with it. Regardless, although of course I can't say for sure what was in Lerner's head, I strongly doubt that he intentionally had Higgins make SO MANY grammatical errors in the lyrics of MY FAIR LADY as a way of purposely pointing out that Higgins may hold others to a higher standard than he holds himself. That's a very charitable interpretation on your part, and I think you're giving Lerner too much credit. I suspect the truth is that Lerner himself was extremely sloppy in regard to correct grammar and usage, presumably because he never learned and/or never digested many of the rules -- and that includes rules that I would hope ALL of us can agree on, such as "If I were..." (correct) rather than "If I was..." (incorrect), along with the debatable ones, such as the one about split infinitives.