If it were something I'd need to trace or just more data, I would have used tcpdump to a file and opened it in Wireshark.
If you're just looking at router advertisements and DHCPv6, not real need to go through all the effort of using Wireshark.
#2 Does the IP Address of the computer running WireShark (or using that command line way), have to be a public IP ?
tcpdump is actually being run on the interface, not on an address. It has to be your WAN interface if you want to see if Comcast is sending out any RAs.