This sounds very interesting and clearly you have given it a lot of thought already. There are a few things I am wondering though.

Are you planning to follow what is shown in Episodes 1-6 as being set in stone of are you happy to do a Lucas and ignore a few bits here and there if it makes it easier to fit your stories?

By your circular theory of Episodes 1-3 being the dark side of the yin/yang, and then Episodes 4-6 being the light side of the circle, then your Episodes 7-9 would focus on that dark spot and moving back into the dark side. If I have understood this correctly doesn't that suggest either your Episode 9 will end on a bad note (like Episode 3) with a bad side victory, or you would have to fit a full cycle (like from Episodes 1-6) into your 3 part story?

This makes me wonder because either you could choose to ignore the ending of RotJ and imagine the Emperor escaped alive and then he can be your black spot bad guy, presumably ending the trilogy with his defeat. Or if you treat the films as absolute then you would need a new enemy, a little like what happened in the Heir to the Empire books.