You must start the betray quest before 18.
....And you don't need to turn off your combat xp if you begin the quest at level 17.
First off, I have a 17 betrayer out there who is, at this moment, waiting for a named mob.
Second, she is 17 with 145 % percent to the next level. While on the quest, if you do reach the required xp for level 18, you will continue to collect xp until you finish the quest. However, you will not "ding" 18 while fighting a named mob or killing those 500 some orcs.
I am unsure of exactly how much more xp I can gather before they makers of the game say, "No. Sorry. No more for you". I will try to keep you posted.
On another note, there is a post for the quests,web-links, how, who and what-not a few pages ahead on the forum. It's a wonderful link to look read and reread while you wait and wait and wait.
Enyoy your search and good luck.