Re: missing interword spaces

    OK. Here's the real problem, cut down to bare minimum.
    The source document is for Plain TeX.

Thanks. I trust Thanh can debug as usual.

    Rather, pdfTeX ought to be identifying that the glue is due to a
    visual space in the content, placed either explicitly in the source,
    or inserted via a \space (or similar) LaTeX or TeX macro.

As you probably know, all such information from the input is lost by the
time of output. All that is seen is globs of glue. There is no way to
know that it came from an original "normal" interword space. Hence all
the heuristics. Hence all the problems.

I doubt there is any final solution to this without significant internal
changes -- all I can think of is to attach to each glue item something
about where it came from. Even the above does not exactly suffice, since
something like \hskip3pt plus1pt minus.5pt is most likely also intended
to be interword glue, whereas \hskip30pt wouldn't be. Except maybe in a
headline. Etc.

It was a design feature of TeX that spaces were not a character. That
was how typesetting worked for 500+ years [Gutenberg to PDF).  It's
highly unfortunate that it's turned into such a massive problem for us
in the last couple of decades. -k