Hello Bill,
You may need to set up static routes for default gateways, more info here:
communities.vmware.com/thread/527804
storagehub.vmware.com/#!/vmware-vsan/vsan-operations-guide/creating-a-static-route-for-vsan-networking-1
Node to Witness uses Unicast, so not sure why you are seeing Multicast alerts - does this specify hosts triggering this alert and if so which?
'Witness not found' likely caused by either Networking config as per the above or incorrect set-up of stretched-cluster, you never elaborated in your other question which I asked you some things on (communities.vmware.com/thread/568798).
Bob