Traya may have no choice, realizing how powerful Kun is (she would no doubt foresee her imminent defeat if she remained on Malachor V) But you make a good point, Kun would probably at least sense Traya coming (although I doubt she'll waste time cutting her way through Massassi - orbital bombardment will soon thin their forces) and maybe prepare some ritual to defeat her.... but what is she blows up his temple? Or waits for him to return (after finding no Traya on Malachor) and sets a trap? That would prevent him from performing any rituals. And I don't think Yavin 4 is a nexus of dark side energy. And if it was Traya could draw on it equally. Just some food for thought.
Go to Yavin? Seriously? Not only will she have to fight through the hordes of Massassi, but she will have to face Exar Kun at the center of his power. I don't see Traya leaving Malachor. She isn't the type of persona that goes about killing whoever she wants. She's a manipulator. She'll send her servants to do it.
Now on the topic of Sion. Are we going to assume that he can come back to life after losing his head? That would be really really lame. And if he could come back, the best he could do is come back as a severed head. Now how useful is he going to be as a severed head?
Or what about the Massassi ripping him to shreds? I have never heard of Sion coming back from the loss of limbs (or his head).
Oh and concerning Sion, I think beheading him would do the trick. But I doubt he's dumb enough to jump into a pit of Massassi.