Dear Maciej,
I wonder whether xBase:
(1) scores relevance of whole matching XML documents and ranks them (in any way) ?
BaseX offers support for the W3C XQuery Full-Text Recommendation, which includes scoring. As you may have seen in the GUI frontend, tf-idf is one (optional) scoring model, which is used for calculating scoring values for query results.
(2) allows to identify (in matching document) XML elements with matched query terms and than navigate to parental/children nodes in XML structure ?
Again (feel free to ask again if I didn't get your point) XQuery should offer you all you need to retrieve the nodes in question.
(3) provides a way to give more weight to some XML element types during relevance scoring ?
That won't be possible, as BaseX works schema-oblivious (no schema information will be evaluated to rank your results).
Hope this helps, Christian