It's difficult to make sense of rules which state "You may not place the meeple on the watchtower itself" in section 2 and then "When a completed road or city contains a meeple on a watchtower..." immediately after.
Decar's interpretation of the rules seem to make more sense to me having played a game using this expansion.
I expect that the intention of the rule is that "a meeple on a watchtower" means "a meeple on a watchtower tile", so the blue meeple counts in my version. Then when that meeple's feature completes, that meeple (and only that meeple, no other meeples in the feature) can score the bonus points for the watchtower.
I think that makes everything straightforward, with no need to track which watchtowers have scored and no possibility of scoring a watchtower twice (unless, of course, there are multiple meeples on a watchtower tile by means of some other expansion mechanic).
The rules could have benefitted a lot from an extra example or two!
Maybe some of our German speakers can help shed some light on things, in case there are translation issues?