There might be an issue in 4.2 that singletons continue to run after merge, there was same issue in 5.1, see https://issues.jboss.org/browse/JBPAPP-6932
Why clusters dont connect up, only reason I can think of is that there is an issue with the network, something wrong with multicasting. You can use a different discovery protocol if UDP and multicasting is a problem for you.
You should check the multicast with this tools [1,2]
If there are lost messages for the MCast test  it will be an indicator for network problems.
It is possible that the configuration or the network hardware is the problem.
 http://www.jgroups.org/manual/html/ch02.html see '2.8. It doesn't work'