To be honest, all these changes to how it counts if you are in, and the need to get into a party, and the high possibility of grief play, I've actually even avoided trying it. Maybe a 'simpler' implementation would be:

-- For each person who joins the caravan, the difficulty is increased (more mobs, higher levels).
-- Each time a person leaves the caravan (talks to NPC to quit), the difficulty is decreased.
-- A counter keeps track of how much a person is involved in the escort (healing, fighting, etc.)
-- At the end, a reward is given only if the person participated sufficiently (say 1 positive action every 5 minutes).

This would mean those who do not actively participate get nothing. Difficulty would only marginally increase due to 'idlers' being present (and the reward would increase as well). The fun bit is, if people join and run to the end, they get nothing but everyone else gets a bit more.

Could also go so far as: You have joined 5 times in a row, but did not participate. You are banned for 48 hours. (Or if they want to be really mean, lock them out of their grand company for a week for bad behaviour).