You want to use the xinclude with DocBook 5.0.

DocBook 5.0 works well with xinclude. There is a version of the DocBook 5.0 schema that includes xinclude functionality. The schema can be found at http://www.docbook.org/xml/5.0/rng/ in the file named: docbookxi.rng.

Namespaces are powerful constructs, but they require attention to detail, especially when you move from a non-namespaced to a namespaced environment.