Hello again - the vent cache was one of the things You have done amazingly to find most of these!
Thank you for the compliments
The textures are from the cancelled Nightwatch mod - so the reference is theirs - but I did recognise it as someplace else myself and put it in. I just love that mod. Adam Foster has made some of the best maps I have ever played.
So - the last thing is on map 14. Kill all the aliens, get Agamemnon (the green suited worker) to open the door. Proceed out until you reach the Military arrival on map 14b. Go back into the parking lot on map 14, and look out the giant door near the security booth
Regarding the last image - I could not figure out that one sorry!
EDIT - and yes, what Marphy found! The game over is very hard to get but was included as a fail safe. The Garg has maybe 9999999 HP or something similarly stupid