Everything I agreed with and (hopefully) fixed has been removed without comment.
This SRFI recommends that in multi-threaded programs the mask be set in the primordial thread before any other threads are created and never changed again.
The above and its repeat for set-current-directory! would be more clear with an Oxford comma at the end, "are created, and never changed again." That helps to emphasize the very important last clause.
Oxford commas are of the form "A, B, and C". They don't justify "A, and B", which is what you're asking for here. Instead, I put the last three words in italics.