it looks like TeX Live does not use "pristine" upstream sources,
Where do you get that idea? I don't make changes to what was released to CTAN (or wherever the original source is, unfortunately not everything is on CTAN, despite our continuing efforts). It is true that we do run the ins's and otherwise rearrange packages from CTAN as needed so that they are actually usable, i.e., in the standard directory locations. However, we don't modify the contents. Of course it would be nice if what was on CTAN was all perfectly arranged and configurable and could be downloaded and used without any changes whatsoever. That is not the reality. It's been talked about for a lot of years. karl
