This is correct as far as I'm aware.Yes, they can, I believe. I don't think the marks have to be taken from four different subjects, only you have to do 4. Though feel free someone to correct me if this is wrong.
However, with MX2, remember that you don't get marks for 2U, only MX1 & MX2
does that mean that if you do 4u eng, and you do better in both the ext's than you do in the normal adv course, and your mark in the adv course is lower than all your other subject marks, that they wont count your adv mark?2 units must be included. Then the take your best 8 units, English or not.