Note that the HTTPD docs explicitly advice to abstain from .htaccess. We still may give it a try and see where it gets us. For sure handling will be easier that way, as everything is selfcontained. We could then do the joining of the fragments as part of the generation and emit it to _site. That way no manual intervention is needed.

Wrt. validation I am not concerned. The merge is trivial, and the fragments should be validated way before (when running the importer and committting the fragments).