Sorry for the issues you are having.
Can you be more specific about which parameters are switching by themselves when loading Presets/Programs? Any specific parameters? Some of the parameters change depending on whether you are recording or if a loop is playing back etc.
I understand your confusion about the mix knob, but that is how it has always worked on our loopers. It’s a mix control between the dry audio and the looper playback, so if there is no looper playback, there is nothing to mix.
Does your mixer have multiple sends? It may be better to use dual mode and use 2 sends with 2 FX paths on the H90.