I'm not sure that I agree that the AO's said you wouldnt pass the interview stage without a GMAT, case in point, I've completed both of my interviews without the GMAT.
For reasons that I'd rather not go into, it would be extremely difficult to complete the GMAT before the deadline suggested by admissions, so my AO said that I might get an extension or a waiver, but after some time I was told that won't happen, so here I am studying for a last minute test.
Where did you get the figure of 640 from? 650 is the bottom of the range INSEAD lists on it's website.