1 - More players will certainly increase the potential to complete the farm fastest, however, the XP and Inf will be distributed equally to each farmer. If a player is dual boxing, there is twice as much work to be done (set up each farmer with their auto attack on the right spot of the map and consolidate their rewards at the end of the farming session).
2 - No, there is a maximum amount of Inf that is earnable per each enemy that is distributed evenly per player.
3 - Another Fire Farming Brute with Burn on auto in a fire farm mission.