1. I don's see a 'Virtualized Devices' Tab - You have to check FGT modeling, something is wrong there, r-click Set Device Mapping
2. Are you sure Dynamic VLAN assignment is available for an SSID in Bridge mode? - Yes they are, basically the AP will tag the user traffic directly and put it on the switch port, you don't have to configure it under SSID.
3. If I don't configure the SSID to use captive Portal in the FGT, which Authentication scheme should be selected when configuring the SSID? - Just leave it open or PSK if you want but put FNAC as DHCP and DNS server, it will put the users in registration VLAN that you can limit access to FNAC only. Registration VLAN should include the network configurations and dhcp relay.
4. I think the 'Logical Network' is associated with the 'Model Configuration' which I mentioned I am not able to see from before. I am not seeing the 'Virtualized Devices' tab either - Yes, but it looks like your FGT is not properly modeled, you should also see the SSID tab