Seems to me that once they knew a nuclear explosion would 'feed' Andromeda, the 'odd man out' hypothesis was no longer relevant and the disarm key should have been moved to Level 1 and used the moment the self destruct countdown commenced. In fact, given that the initiation sequence was automated, the odd man out should always have been on Level 1.

Maybe so, but movies like this are all about Because Plot/Because Drama.

Actually, I remember from reading the book that the detonation was just the final step in the self-destruct, the process in the novel was actually stopped at like 5 minutes to 0 or something, but that was still critical because just before the bomb was to go off, the air from the facility would be pumped out to make the explosion more effective somehow. So the people were going to suffocate even before the bomb went off.

They didn't do it that way in the movie, though, Because Drama.

I agree. With SF, more drama usually means less science, and vice versa.

Something I forgot before, I seem to recall that in the book, only Hall could use the key, because it sensed his body capacitance or something.