You have good questions!
1) Alternate the change of inserts each month. For example, month1 replace just the sponge, month2 replace the carbon only, month3 replace sponge only, etc. The reason for this is that beneficial bacteria will eventually colonize every surface area in the filter housing, sponge, carbon, etc. This bacteria breaks down the turtles waste into less toxic substances (waste = ammonia --> nitrite --> nitrate). When you replace one of the filter inserts, you are removing a large chunk of the beneficial bacteria. The good thing is that if there is still a healthy colony in the other insert, it will quickly colonize the new replacement insert.
2) Rinsing the inserts helps unclog them from all the solid waste that the filter picks up. You want to be careful not to rinse out too much of the bacteria though. Tap water may contain chlorine which is why you don't want to rinse with that. The best thing to do is scoop out some water from the tank, rinse the inserts gently in there. Dump out the dirty water down the drain and replace the same amount with new conditioned fresh water.
3) You should do regular partial water changes on your turtles tank. This should be roughly 20% of the water volume every 2 weeks. Use a siphon hose and bucket or get a Python -->
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=3910&N=2004+113164 Remember the beneficial bacteria reduce the waste from its ammonia form down to less toxic nitrates. But the nitrates still accumulate over time and need to be kept down. The best way to do this is through water changes.
4) Adding water to replace evaporation is not the same as a water change. In fact it can cause more problems if your tap water contains any nutients, heavy metals or other elements. When water evaporates from the tank, it is in the form of pure H2O. Everything else remains in the tank. As you add more tap water to make up for the evaporation, you are also adding more of whatever else may be in your tap water. As the H20 continues to evaporate, the extra stuff from the tap water remains. Over time, these things will accumulate to harmful levels. Regular partial water changes slows down this trend, but the best way to deal with it is to prevent the accumulation altogether. To do this, you need to use distilled water (pure H2O) to make up for evaporation. If you have a reverse osmosis filter, water from that is just as good (RO filters produce almost 99% pure water).
This kind of water mainenance might be overkill since turtles are tough little creatures and aren't as affected by water chemistry as some delicate fish species can be, but if your tap water has problems, it will become greatly magnified in a captive aquarium over time. Something to be aware of. Problems with water can also be staved off by doing a large water change (50% to 75%) every 6 months or so.
Hope this helps.
