1. Yes, but actually the IP address is accessible on lan0 and wan0, since those are just switchports and the IP address is for management.
2. Yes, some default gateways. routing, policy based routing or other type of redirect is needed to get traffic from the lan to go to the virtual lan0 and traffic from the wan to the wan0 address.
3. Yes, while not an official 'mode', in this scenario optimization is not possible, but the device is on the network for configuration purposes.