I tried both your suggestions and indeed, as you said, using that little script to create a modulation source is better than using a QC as the knob in the MP will display the correct value of the pitch modulation and not just a generic value from 1 to 100.
But now I encounter another problem: let’s say I have a main layer that contains two sub-layers (A and B) and each sub-layer contain some samples. What I try to do is having 3 knobs on the macro page (knob 1, 2 and 3). Knob 1 to control the pitch of layer A, Knob 2 to control the pitch of layer B and Knob 3 (and here is the issue) to control both the pitch of layer A and layer B but in a relative way. So for example if Knob 1 is set to -3st and Knob 2 to +3st, when Knob 3 is set to +4st I expect to get a +1st pitch modulation for layer A and +7st pitch modulation for layer B.
Is it possible to accomplish this somehow?