The +-90° phase shifts are super easy to implement using the "Phase Bug". I just checked and Phase bug is no longer available for download but I have it if anyone needs it. Audition has its own phase shift function built in but I don't think that it is as convenient as using Phase Bug.
To QS encode you add the signals, front pair and back pair together in the proper proportion, everything stays in phase. Then use the bug on the back pair. That was the big advantage of QS, no cancellation during encoding as everything stays in phase during the encode process.
It just occurred to me that the same process could be used with the EV and DY coefficients to make an encode immune to cancellation encode effects, plus the output phase would be correct when using a QS decoder!
https://www.kvraudio.com/product/phasebug_by_betabugsaudio#