It is recommended to only enable Netflow on the Multilink interace and not the member interfaces. Also is this router performing NAT? If it is then you would want to capture Netflow form the LAN interface in order to see the internal private IP addresses.
Hope this helps.
Should we need to still specify the source as in this case:
ip flow-export source Multilink1
or should it be
ip flow-export source loopback1
or no source???
It is best practice to always specify a source ( interface with IP address Orion NPM is managing device by) If a source is not specified the device will choose one and it is not always the one with IP address in Orion NPM for the device