I got it working. You nudges and good questions got me on the right track.
That Instance XPath property was the key to getting the right syntax in the xpath query. My final query looked like this if anyone is interested.
AddNoteRequest.text = xpath(WachNewAccount,"string(/*[local-name()='Header']/*[local-name()='AccountNew']/*[local-name()='NoteNew'][" + counterStr + "]/@*[local-name()='Narrative'])");
Thanks!