The question is not if NS sso supports Moodle, but if Moodle supports NS authentication. The answer is yes and can be found in Moodle documentation: https://docs.moodle.org/24/en/Active_Directory
For your 2nd question: just configure 1 interface as green and do not install the firewall/UTM module. You can decide to hand over DHCP and DNS to NethServer instead of the mikrotik router. If you need the proxy service, I would consider a second interface since a single interface can be bypassed. In the case of a second interface you can ‘double nat’ NethServer behind the mikrotik.