Frank,
Yes, your "routing idea" is ok, but it is important that the panning pot circuit not leak any signal, example to the right channel when the pan are in full left possition.
If there are a "virtual summing mix buss" with summing point in to the (-minus) inverted input on the op-amp, (or on some Soundcraft consoles ther can be a "base grounded" transistor before the op-amp) you not need to change the summing resistors, because the virtual input will adjust it self.
When you get -6dB loss in the summing point every time you double the number of resistror, you get +6dB gain from the invertet summing amp.
So long there are a NE-5534/32 as mix buss amp, there are no bigger problem with the noise, but you can get lower noise if you use a ADI AD-797, but this are only in single version, and can also be a bit instable with out some modification on the pc-card layout and so on.
(not need to change op-amp, if there are a "basegrounded transistor as a front-end amp)
So you can connect all "same" mix busses on the motherbord or ribbon cable harness together, but use only one of the consoles mix-amps for each mix buss rail at same time.
It is very, very important to connect a extremely good ground connection between the consoles, and not mix-up the different ground system, if there are separate grounding for power, mix busses, cable screens, frame/chassis, etc.
--Bo