Your setup is not clear to me from your description, for me, an instance and a realm is the same, a cluster consistes of different realms. Inside a cluster the nodes should be identically, having different configured nodes/reals inside a cluster does not work imho.
Why do you have a loadbalancer between applications and the UM server? The connecitvity should be done by the client libraries as well as the automatic failover.
#Integration-Server-and-ESB#webMethods#Universal-Messaging-Broker