I don't think Nihilus could actually end the Force. Then again, the whole "Wound in the Force" concept really wasn't fleshed out properly.
She probably did lie to get the Exile to kill her as the Exile's "final test". Unfortunately, the Revan novel revealed that it was the final test for the Exile to go into the Unknown Regions and lose horribly to Nyriss before being backstabbed by Scourge.
Well I think its not a case of ending the Force, but cutting everyone else from it i.e. to create wound so powerful that the echoes would deafen them to the Force (like Malachor did to the Exile) or kill them. And seeing as Nihilus was going about creating wounds it stands to reason he could achieve that, if such a thing were possible.Yeah, I'd also speculate that she did it to sever the connection between them, with Vrook saying that one way to sever a bond is if the relationship between the bonded undergoes radical change, betraying the Exile would have done that.
And true, though what Traya wanted her to achieve exactly is ambiguous, I'm giving the impression she didn't really care, and was satisfied having created a student that vindicated her teachings and cleaned up her mess before kicking the bucket.