I was able to give the truss rod a tweak to decrease the relief using my stewmac tool (without taking the neck off) and it didn't feel any different to a normal truss rod, so I think it is probably okay functionally, however I just don't know enough about what is inside the neck causing the ajustment nut to be recessed so much.
My main reason to return the bass was actually that it was listed as a 1971 mustang, but the serial number indicated that it was not quite that old, and there were other clues to back this up. I believe this was a genuine mistake, and the shop owner did seem genuinely concerned that I had been let down, so I won't name the shop publicly. It is a terrific bass otherwise — certainly on a par with my perfectly set-up twin JMJ mustangs, but with a lighter swamp ash body and some good-looking genuine mojo.