Tonight, I got the XStreamDB EOAdaptor firing on almost all cylinders. It has been able to query and retrieve XML documents for quite some time now, but it can now also update (replace existing documents with edited versions) and insert new documents. It's not extremely tested yet, so there may be some pitfalls or errors (likely an error or two, or more likely some overgeneralized assumption or the like).
It's got a hard-coded reference to the database and root at the moment, but that won't take long to switch to code that pulls that from the EOModel for the Entity in question. Also, there isn't a way to delete documents via the adaptor - but for now I'm more than fine with hitting XStreamDB Explorer to do that manually...
I plan on updating the Sourceforge project sometime next week, after I polish the adaptor a bit more, and fix JavaEOXMLSupport to enable addition of new elements to a document...
What a relief this is. I've been feeling like a total bumbling moron for the past few weeks. And, with this working, I can get back to work on the rest of the Pachyderm...