After some more tests, it seems now clear that the Multi Mixer concerns only the Main Output and has no effect on the Aux Outputs. So I have redrawn the diagrams to reflect this.
It is also clear that the FX do reach the Aux outputs.
So I think that the Serial Mode diagram should be rather faithful now.
As for the Parallel mode, some things are still a bit fuzzy.
There remains the control of the return values of FXs (Program page, Mixer Panel).
I placed them just after the FXs.
Aren't they redundant with Fxs levels (located on the same page)?
They seem to have the same impact on the audio effects as the Fxs levels, with the exception that they work globally on the four Out1,...,Out4 (and correspond to the three buttons of the front panel).