Hi Xave, I can give a little info, but it's hard to remember everything. For stage two what they really test is your ability to multi-task. So you'll do things like pick true or false on if an image on the screen meets certain criteria. You'll also do a radar test where you have to stop planes from crashing into each other while also solving math problems. I'm not sure if there is a way to practice beforehand, but before each test there are practice questions which you can do as many times as you want before you begin, that's what I did to help me for each test.
As for your other information I can't answer it, I have no idea on the statistics for filtering.