1. The built in FortiWifi does not support bridge mode SSID, what bridge mode SSIDs really mean is to just be on the same network as the interface.. but in the FortiWifi radio interface case it doesn't have a network. In order to bridge you would have to use a tunnel mode SSID and create a software switch to bridge two interfaces(the LAN and that SSID). So, this may require reconfiguring you LAN as well. The software switch will be the interface with the IP/netmask/dhecp server/etc, anything adding into will need to be 0.0.0.0/0.0.0.0 with no references.
2. Built in FortiWifi is dual band but only single radio. So since it is single radio it can only do 1 at a time, you need to pick one or the other.