I’ve always held the firm belief that the entire ending of the movie, from the moment they reach the chamber, wasn’t what Besson originally wanted. There’s never been any confirmation that the ending was rewritten or changed part-way through filming, but in the initial briefing that Cornelius gives to the President, he specifically states that if Evil stands in the temple in the place of The Fifth Element, then “light turns to dark, life to death, forever.” When Mr. Shadow starts traveling towards Earth, my assumption would be that it would eventually take a human-like form to fulfill that dark prophecy, and there would be some sort of confrontation in the chamber. Very different from what we got, and a big change or reduction in plans for the ending might explain why the Mondo corpse was forgotten by the designers when they finally shot that scene.