Yes, James Haughton is the longest reigning series champion, holding the title for 364 days between the series 81 and 82 grand finals. The next two are series 53 champion John Mayhew (329 days) and series 6 champion Darryl Francis (274 days).
Any calculation involving what seed people are is hard because the database doesn't know things like how the seeding rules have changed over the years, but I don't think we'd find a better answer than Luke Johnson-Davies as you say. He started series 82 having already won 5 games on 2nd January 2020, and kept that position all series. Series 83 started on 4th January 2021, so depending on whether you think someone ceases to be #1 seed at the end of their series or at the start of the next series, Luke was #1 seed in series 82 for just under or just over a year.
I'm assuming someone becomes "champion" when they win their first prelim, and cease to be "champion" when any prelim is played which that player does not win. By this definition, Clive Freedman won his first game on 12th March 1985 in the last prelim of series 6, and remained the champion until the game after his eighth win 226 days later on 24th October 1985, which was a few episodes into series 7.
Thanks to Gevin for pointing out the answer to the last question five months ago.