a) you need the water on when flushing - all you are doing is creating an easier route for the tap water to flow, which happens to be over the surface of the membranes thus flushing away any debris.
b)You can get replacement filters from us/gardiners/freshwater/surecleansystems/cleantech - just about anyone.
c)the first filter is the sediment filter, it will start to turn brown quite quickly but will remain effective for quite a while depending on the level of sediment in your tap water, my advice is to replace it once every 4/6 weeks - the sediment filters are only a few pounds each so its not an expensive filter to replace. As a general guide replace the secondary filter (carbon filter) every other time you change the sediment filter, that is, if you change the sediment filter every 4 weeks then change the carbon filter every 8 weeks. It is imperitive that you change the carbon filter regularly as it removes chlorine from the water - chlorine burns the surface of the membranes rendering them less effective.