welp there weren't any responses, but i was able to figure it out anyway.
For the master and secondary domain issue; it's as easy as moving all the VMs you want off the master, then detaching it, the second domain becomes the new master automatically.
For the Cluster A to B move; I shut down all the VMs, created a temporary secondary data domain (with no data), then was able to put the master into maintenance mode, detach it, and the temporary secondary data domain became the new master.
I then attached Cluster A's old master domain to Cluster B, and was able to bring up all the VMs immediately. This whole process took less than an hour, mostly because I spent time poking at stuff, but realistically took just 15 mins.
I also learned a lot about moving VMs to a new NFS, first step being copy all the templates and for thin clients their referenced disks to the new NFS, then shut down the VMs and move their disks. All real easy through the UI.