1. IDK, but it doesn't matter because the price of similar items is what you should use to sell your item
2. I don't know the math on this, but I always do a 6 Hour auction when I will be able to check the GTN and/or in game email for the entire time period. If the items are priced right they should sell within 2-3 hours during normal play times. If i'm going to be logged out for more than 6 hours, I'll change the auction time to 12 Hours. If I will be logged out for more than 12, then 1 Day, etc.
-Keep in mind that you get the deposit back when your item either is sold or the listing expires. You only lose the deposit when you manually remove the item. Unless I'm going to bump up the price quite a bit, I will typically never remove an item.
3. I know for sure that the fleet and dromund kaas share the same GTN, don't know for sure about any others.
4. GTN commission is 3%.
I hope this was more useful than the stupid link that the customer service droid gave you. If you're paying attention BW, you could have at least told them about the 3% commission since its a static amount(The commission rate should probably be in the manual too.