Okay, I finally ran the “False Emperor” flashpoint yesterday (avoiding it since 2011) to put Onslaught and 6.3 into context.
So, my question is: How did he come back? He didn’t die in a convenient explosion where we never recovered the body (which I expected based on the post KOTET dialogue). I beat him to death with my lightsabers.
So, unless he has some sort of Darth Sion vibe from KOTOR 2, it seems problematic.