>                                but instead of arbitrary adding a 0.25em
> before and 1em after the punctuation mark you should use the real nnbsp
> (U+202F) before and real normal space (U+0020) after.

 I don't think so.  Space characters don't mix very well with TeX glue
and should best be avoided, generally speaking.  In particular, all
inter-word spaces that are input in the TeX source as one or more of
U+0020 are simply ignored, and replaced by normal inter-word glue, with
its appropriate stretchability and shrinkability.  This has always been
the case in TeX and is not going to change.  All other types of Unicode
spaces should really, in my opinion, be processed in the same way, while
respecting their additional properties in the case of non-breakable
spaces, for instance.