update:
toyota service looked at the lamp hole, and told me that the screw was not original, that the head had been ground down. they couldn't tell me if anything was missing otherwise.
went to two body shops (one at a toyota dealership). both seemed to think that the headliner slots were slightly damaged and that was preventing a tight installation. i disagreed. the headliner is too flimsy in those spots to snap into and be supported.
i finally decided to disassemble the rear lamp which is identical to middle one.
lo and behold, the metal crossmember does in fact have slots specifically for the corner hooks on the assembly to snap into. that provides the secure mounting.
so, the middle one is incorrect and missing those entirely.
i now have a clearer idea of what happened. when the roof panel was replaced, of course the whole headliner had to be removed. the crossmembers were most likely all replaced ( i have the insurance write up; the estimate nearly doubled in cost from the initial appraisal). my theory is that someone attached the wrong style crossmember over that lamp area. when they had the whole headliner reinstalled, and were reattaching the lamps, they discovered that the error and improvised by drilling the screw into the metal and suspending the lamp that way.
unfortunately, the only real way to correct this is to disassemble the whole headliner, and replace the crossmember with the right part. all that so i can tightly mount that lamp.
see picture below of the rear crossmember with the wholes available for the corner hooks.
