Thank you! Agreed on all points.
I initially did call the variable `include' but Emacs scheme-mode highlighted it using a keyword face, which made me worry if I shouldn't choose something else in case include is some kind of reserved word.
I didn't know about `with-directory-excursion' before but it's clearly preferable. I got the idea to use `chdir' from looking at other packages (it's used many hundreds of places.)