I have not followed all the details of your debate but I get the
impression that you may be wrong about how \vspace interacts with \addvspace.
This is best described as `no interaction', since \vspace and
relatives all `end with'
The purpose of this is that, in LaTeX, _only_ those spaces
added by \addvspace should interact (and with \addpenalty);
all other spaces are irrelevant.
Of course this does not work in general because TeX skips get added by
all sorts of internal processes; on the whole, the LaTeX code tries to
pretend those do not exist.