For example if I have a MIDI pedalboard with 2 expression pedals + 6 MIDI CC buttons, can I, IN THE SAME PRESET, control: Volume, whammy and switch on or off every effect block?
You can assign as many parameters to a modifier as you want, in fact you could assign all the controllable parameters to one expression pedal. Of course, these parameters will all get the same input value from the modifier. But, you can decide for each parameter how you want it to respond to the modifier input.
This is because the mapping from modifier to the controlled parameter can be set individually for each controlled parameter. When you are on a parameter to which you have just assigned a modifier, press edit (PUSH ENTER). Here you can set a response curve for the modifier with minimum, mid and maximum values.
Min Out 10%
Mid Out 50%
Max Out 90%
As mentioned above, the response values can be set individually for each controlled parameter in the G-Major. This makes it possible to have more parameters reacting differently to the same modifier.
The answer to your second question is: Yes, if your pedal board has these possibilities, you could at the same time control volume and whammy, and switch every effect block on or off.