Mitose used the "N" spelling all through his book, even on later editions. Intros were added to later editions, so the retention of the "N" spelling could have been changed, but it was not. The addition of intros meant that the later editions were not "exact reproductions" of the original edition, so changing the spelling would not have been a preservation issue.
Mitose also used the "N" spelling all through his later book, "In Search of Kenpo". Same printer, same error? I don't think so.
If you run the word "Kempo" through the Japanese dictionary on the web, it comes up something like "incorrect romanization", while "Kenpo" is listed as a Chinese-based martial art.
My opinion (2% of $1): spell it as you choose, pronounce it as you choose, practice it as you choose. What the heck.