When the load routines for nXhtml discovere that you are using a version of
Emacs that includes nXml it will use the version that is included in
Emacs. (Otherwise it uses the version that comes with nXhtml and is the
original version from James Clark.)
If there is, what you probably need to do is make sure your locally
installed one appears in the load-path before the once bundled with
emacs.
So you should not do this. Please see above.
Hmm. While I see your point, I would still be suspicious that if you
have nxhtml-mode and iff it comes with nxml-mode, then it would be quite
feasible that depending on load-path settings, you may pick up parts
from the different versions of nxml-mode, which in turn could result in
inconsistencies if there has been changes made to one version and not
the other.