You are guaranteed that the Channel is connected by the time any cache-level component using JGroupsTransport starts. So the moment the listener is added, it can receive view updates.

I don't think there is any guarantee about JGroups view updates arriving on the same thread. On members other than the coordinator, view updates are processed as in-order messages from the coordinator, so they will be processed on the same thread (unless perhaps the coordinator just changed...) On the coordinator I'm not sure if JGroups does anything to serialize view updates.