Great question. I look forward to hearing what people suggest.
> eXide looks great for XQuery editing but seems to offer less than
> CodeMirror when it comes to XML.
I'd dispute that eXide isn't good for editing XML. It's quite
serviceable. I agree that eXide lacks #4 and #5 in your list, but
could you elaborate what CodeMirror offers that eXide lacks?
Also, in your list of requirements you might want to specify what size
range of XML file you want to be able to edit in the browser.