Even if she gets her hands on the Saint Killer, it's still just a sword (probably). Someone still has to get past all of her magic to get close enough to cut her with it. I can imagine someone getting cocky thinking they can kill her easily because they have the sword, and then she just blasts...
Nope. Nuh-uh. Don't like brainwashing. That said, I fully expect him to stab her with the Deadly Serpent Venom and then revive her with the Lifeblood Vial, and that will somehow undo the brainwashing.
I mean, in general, people can be friends without being/becoming lovers. We always knew that they would fall in love with each other because the manga is tagged "Romance."