Fee is the ante. This is what you ultimately bet against. I'm not sure about the win fee.

Too high for win, on the high end for sell but doable. I'd recommend lowering the win fee to perhaps half of this.

No, but don't forget to charge withdrawal fees. I haven't checked what you have right now.

Re: your "user created markets" idea. I think this is a good idea. Since you can't prevent someone opening a second account (as you do yourself with the "bot") just let the market creator enter on their markets? Just have to know who created a market so that we can spot fishy markets