You can do your Betrayal quests from any point between levels 15 to 17, as I recall. Once you start it, you're level capped at 17 until it's finished.
You may be able to betray twice, but given the amazing amount of work and XP you'll be getting by doing the quest itself it's likely you'll be at 17 and 220% into the next level, which means you'd autoding upon completion of it and be unable to betray again.
It won't ***** you, since you have to do it before selecting your subclass; therefore, if you were a Crusader in Freeport, and therefore going to become a SK, you are now a Crusader in Qeynos post-betrayal, and going to become a Paladin at 20.