Is Empress Cygnus semi-fixed?

One more question that has yet to be answered, if anyone's still checking up on this thread (That knows this for sure).

I read that it spawns all the main instructors at the same time, but when does that happen?

I'm guessing when the Empress' HP reaches 1/2? Would I be correct?

Or is it as soon as you kill all the 600m instructors, or a combination of both?

Do you need the mini shinsoo to die before Empress summons the 1.65b instructors, or can you just continue attacking empress.

Thanks again for the help/info. Were just going to tank it out if no one can answer, because we thought shinsoo spawning was the glitch, but it seems unrelated now.