9 Ball

If the other player does not have to call any shots, and the called 8 is for you to win, I would count that as a pocketed shot and you continue shooting. The "called 8" is for the win only. If it goes the other way, you would basically be saying "I have to call the 8 for it to count as a shot but the other guy does not" which is not very fair especially since you are getting the handicap.

Call 8

Yes, "called 8" is a "win" ball... if it goes in an uncalled pocket, you continue on the 9.
If your opponent made the 8 in a pocket he wasn't shooting at, he would continue just the same.