> Those bolts may have very well been in place and passed an inspection during install, but rattled loose and fell out (and then subsequently were blown out of the plane when the plug failed.)

The bolts the NTSB can't find are not the ones that have been found to be loose. The bolts they can't find are the ones with the castellated nuts and cotter pin. Very hard to imagine those vibrating loose. And for the door to transit in an upward direction it would really require all four of those to fail or for them to not exist.

https://www.youtube.com/watch?v=maLBGFYl9_o at around the 7:26 mark has a great explanation of how the door is secured and it seems like as long as there is at least one of the four bolts installed the door can't move.