When my component files were in the same directory as my component files,
the line
%! TeX root = prd_whatever
allowed processing the component file from the component file. (I use
TeXShop for this.)

The project structure has been changed: the component files are in a
subdirectory of the directory that has the prd file. Now,
%! TeX root = prd_whatever
no longer works. (E.g., it cannot find the environment files called in the
prd file.)

Is there a way to restore processing from the component file. My
experiments are near to wearing out the “error” message/page.

Alan