Your Lewis quote contradicts your very first paragraph, as the quote argues languages aren't like logic and arithmetic, and in your first paragraph you assume languages are like logic and arithmetic enough to suffer from the same incompleteness problem (if I remember correctly only systems that include arithmetic can't be proven consistent?).
"More than one person is" is correct not because of any systematic reason, but because of euphony - it is more pleasing to the ear than "more than one person are". That, and whether the communication was effective, are the rulers of languages. That's how we got to where we are and how we go to where we are going. Not with central bodies regulating languages and not with language authorities we must emulate or check our usage against.
> Complaints about the "mistakes" of great authors are generally [...]
Yes, a lot of people have trouble accepting that even the greats make mistakes.