I wrestled with XMLType for a long time, and finally abandoned the whole thing. I suggest you to try another XML:DB.
eXist has been working perfectly for me, try one of the latest snapshots. You could embeded it into JBoss, as an MBean.
Oracle XMLDB supports XPath only, and is kind of broken...
eXist has support for XPath and XQuery. Try to express you query in XQuery instead of XPath fragments scattered, and see the difference.