Once you begin betrayal, you may not become level 18, period. I'm not sure what point that actually begins with, whether you have to be killed by your monarch before you become considered a traitor, but it isn't doable. Once you hit 18, you can't betray. Once you betray, you can't hit 18. You can't become your subclass until 18. One of those vicious circle things.
Freeport classes are not designed for Qeynos, and vice versa. If you manage to find some glitch that let you do the unintended, they'd thank you for finding a bug, fix it, and roll your character back.