A few ideas on getting a mono/summed signal from your keyboards...
1) Use the L/Mono output only
2) Use stereo outs, pan center, use TRS > TS cable(s)
3) Use a Y-cord (actually, highly "unrecommended;" you should use this type of cable to split a signal, but not sum. I only include it because thousands of people worldwide do this anyway, regardless of consequences)
4) Use TRS cables with mono (TS) adaptors
5) Use inputs in stereo; sum to mono in one track of your DAW (Cubase)
6) Use a keyboard sub-mixer to sum the signals before inputting to one channel of your mixer
7) Use Bus/Aux Send set-up to apply effects to a single keyboard signal, to do your parallel processing with only one keyboard output
TBH, I think you might be better served by using your DAW to create your parallel processing set-up for EQ's and FX, rather than using your limited amount of inputs on the mixer for that task. When using one signal and sending it to 2 or 4 separate effects groups, you won't have to worry about the volume thing so much. When you bring faders up on the DAW on all keyboard tracks, you will still have the 6dB increase (that's just a given), but you will be able to control it easier because the stack of sounds is coming from a single source (your one initial mono keyboard output). You can even group the faders together from all of the various synth channels so that once you adjust them in a sub-mix that you like, when you move one fader, they all move as a group.
I hope that helps. I think I understand what you're doing; I just probably wouldn't do it that way myself...
GJ