The Empress Wave was a plot device to uplift the role of psionics within the setting. It had not been fully fleshed out at launch, if Dave Nilsen or Frank Chadwick ever really knew what caused it in universe they have never said in any interview I have seen.
The original moved at lightspeed, not FTL, but because Don got his maths wrong he thought the wave had to be FTL to arrive in the setting as per TNE - it didn't. The problem was that the Zhodani would have already encountered the issues caused by the wave during their core expeditions, and for the TNE timeline then the wave must already be in Zhodani Consulate space in 1105... neither ow which was true for 1105 as detailed in CT or MT.
I just hope that in this timeline the Zhodani win the FFW, or at least get to Rhylanor and press the reality engineering big red button to remove the wave from now on.
Like I said, any referee who wants to explore its concepts can read up on Dave Nilsen's revelations about the wave, MWM/Don re-imagining the wave and introduce it to theri univers in a way that makes sense to me.
it may even be worth a JTAS article on the different variations of the wave, different origin stories explaining where it came from why and what it does...