Before I get into full debate mode, I will say that the HK-50 assassination scenario will not work. Not because of the Sith in the Trayus Academy, but because of Traya's technique in Force Cloak.
Traya is blind. She cannot sense the droids with the Force. A HK-50 assassination will work.
Note that HK-47's and T3's fates were unknown to Traya, she could not sense their future. Which means she will not be able to foresee a HK-50 attack. Nor will she be able to sense their presense, and they're assassin droids. They won't be giving anything away with sound, etc. She can't see them. She's blind in more ways than one.
Also, pre-cog "spidey-sense" works seconds before danger, not as it is approaching. So even if she force-cloaked, she'd already have blaster bolts flying at her from HKs she didn't even know were there.