How about countback like they have in the predictions game in the cycling news mag. 1 point for 1st, 2 for 2nd and so on. If the rider DNFs then you get points that correspond to the number of entrants (As if youd finished last). Lowest score wins, which would always give us a winner. Any ties being decided on who entered first.
On when should the race be closed, why not stop it once 50km to go occurs. Then its a proper guess as lots happens in the last hour but allows everyone to get an entry in.