Are the minor revisions "end of the world" types of revisions? Could the CS file the mylar with their corrections hand written in ink on the mylar? If you don't find the guy, that might be the best option. They could put a note on the map that any hand written comments are made by the CS after attempts to find the surveyor of record failed...or something like that.
Could be the broker got their fee but never paid the surveyor, so he never produced a finalized map. But that doesn't explain the surveyor's disappearance.