I just got the first switch described above completed last night (had a delay due to a bad switch and no spares), and external is really really weird. If I hold a key on the keyboard, it works as expected. If not, I get whatever the last note the external played repeated with the incoming gates.
This prompted further review of the mods and the circuit and I think I realize what the problem is, and the switch model I describe above will absolutely NOT work.
The repeat and S&H gates are integral to each voice ( first of all ) and so independent. And the S&H gate in particular is just the internal hold of the last key's voltage. Lack of familiarity with the keyboard had me confused on that point.
So for external to work correctly, *all three gates* have to be hooked in for the voice in question.
So the right way to wire this for independent upper/lower external control is two 4PDT switches, one for each voice. Each switch then has CV, key gate, rpt gate, and S&H gate enable or disable.
I plead unusual architecture and weird gate routing in my defense for not understanding how it works prior to this.
EDIT: And I still have it slightly wrong there. The S&H is per voice, but the repeat gate is not. So I've wired that so that the non-mod internal gate wiring goes to both of the 4 pole switches for the "off" position, and the center on that pole goes to an additional SPDT switch to select whether the repeat gate is fed from the upper or lower voice.