1,2. It is by default OpenWrt.
3. Yes, you can use USB hub. You don’t need a powered-usb hub if your hub don’t requires separate power
4. Yes. a) b) c) should works fine. d) we have no experience. But it should work as long as you have the right driver.
5. Which arduino USB board?
6. Yes. This is what we did in Domino Qi. Arduino is connected to Domino Core via serial and GPIOs.