Ranks in previous matches

It's a custom Valve's variation of the Elo Ranking System and no official documentation has been released.

By my guesstimation that demo is gone forever. The Recent Matches tab displays demoes stored on a cloud, and most probably they are simply popped/deleted from the list as it overflows - Would be costly to allow an unlimited amount of storage space per user.

Your best bet would be to contact valve. I had no luck figuring out what they system entailed through google.

EDIT: It would seem that the .dem files might be time-based stored, not just popped when > num. Read more: How can I get a match demo that I can't download?

Might be that they do both! But if it's time-based, it could be possible if you contact valve that it is still in their systems and that they could give it to you.