Hi Florian,
On your first point, regarding also affecting the VCA, the Polysix schematic makes it look harder to affect the VCA envelope as the VCA CV comes direct off the Envelope IC (via a solid state switch). In other words, the envelope for the VCA never leaves the per-voice section of the circuit. This is in contrast to how the envelope is sent way away from the per-voice section of the circuit, it is multiplexed, and is then modified by the "EG Int" processing. If I target just the VCF envelope, I only need one circuit (not six) and it's a simpler modification. Maybe in the future, once I gain confidence, I can also make the VCA envelope be velocity sensitive.
On your second point, regarding how the "EG Int" also adds an offset to the VCF cutoff, I'm all over that. I detected it pretty quickly after getting the Polysix and I thought that it was a nice touch. My plan is to affect attenuate the envelope itself and NOT modify how the EG Int is applied. As a result, the way that the EG Int applies the offset will be unaffected by the velocity. I think that this will result in the most natural sound.
Chip