Sorry, you can Betray only once. I've spent some time in Freeport, didn't like it at all. You'll find Qeynos much more pleasing and easier to get around.
You can find details of Path of the Conjurer
here.
I did the class quest so long ago I don't remember if I was given the choice of Necromancer/Conjurer, or just Conjurer.
If you absolutely must stay in Freeport, you might consider sneaking into South Qeynos to talk to the NPC. I wonder if that would work...