Btw, please try not to double/triple post. Try to keep all your posts in one (use the modify button).
500 is a lot of interviews it would be 300-350 400 at the most. If they took 500 interviews out of 800 what is the point some of them would be below average. Id say less then 400.
They did interviews for about 6 days, so that's around 80 students everyday (if we use 500). The groups contain 5 people, so that's 16 groups per day, which means that it's feasible to interview 500 people.
After the interview do they look at your test results or is it only for the cut off for interview shortlisted students.
Since you got an interview, it means that you've scored in the top half in comparison to other students. For the interviews, I'm pretty sure they shortlist based on what you say and do (puzzles and individual interview).