This might be fun.
I don’t have any thoughts on the creative questions.. .but the technical ones?
What is your audio source? I saw you mentioned CDJs.. but which ones? Of course, if it is a computer.. this can be easy.
Also- try some things: What happens if you just plug the USB from your mixer into the H9? (I DON’T think that would work.. but what if it did!) Set up the H9 to recieve MIDI clock, and see if that mixer can control tempo via tap.
If that doesn’t work, you mignt need to sync via a computer.
You should be able to handle this on your mixer.
If you are using the Aux sends.. just dial in the return signal with the upper right knob. Notably- I would setup the H9 in ‘killdry’ mode, with returns being FX only.
IF that isn’t working.. maybe run the H9 output into one of your Mic channels? then you would have independent control of the returns.
Let us know how it turns out!