nobody to answer/advice me on this isolation problem?
your post doesnt indicate if your problem got solved or not. anyway, the first 2 settings that you mentioned(UseJbossWeBLoader and Java2ClassLoadingComplianace) are at the level of jboss-tomcat integration, which means all the wars installed in ur server will go by this configuration.
You can specifically override this behaviour by setting the Java2ClassLoadingCompliance in ur jboss-web.xml for *each* war in ur server. If you set this property to be true, Jboss will look up the parent ClassLoaders to look for the package(and the class) before it looks up the child repositories. For e.g. if you had a commons-collections in the jbossweb-tomcat*.sar and also a commons-collections in ur war file, the one in jboss-tomcat.sar will take preference over yours if you set Java2ClassLoading property to be true.