Which you use varies based on the number of magic dice you roll IIRC. Although I think that the highest result on the lower two tables is "roll again on the higher table".
Yep, that's right. A roll of 96-100 on the Minor Miscast table is to roll on the Major Miscast table, and a 96-00 on the Major table is to roll on the Catastrophic Miscast table.
And if you roll between 81-90 or 91-00 on the Catastrophic table, the GM hands you a new character sheet.