Some wins, still some problems.
I investigated this coat hanger mod, and mix bus - mod. The coat hanger is a a wire that effectively takes to 0v ground off the mix bus - on pin 10 of your module lower edge connector, and re-routes it to pin 78 of the lower edge connector. The trace is then cut, so the mix bus - is independent of 0v ground.
There is also the "pseduo" balancing of the quad bus (?) by adding four vertical resistors on pins 69, 71, 73 and 76 of the upper edge connector, and cutting the ground trace by drilling out the pad on the underside.
By reversing this, I was able to get the modules to function correctly. However, I still have the summing issue on the 26 card, so to work around this, I can route all channels to 31-32 mix bus, then patch the group out to the pre VCA insert return. Ugly work around for now, but at least I can start to trouble shoot other areas.
Coathanger wire cut at pin 10 and routed to pin 78
Coathanger wire start, near "Ready Tape" switch.
Coathanger at pin 78
Coathanger at pin 10 & trace cut.
Pseudo balancing resistors
Traces cut at pseudo balancing resistors on underside