Wow, thanks to everyone for the very useful info! Especially
@Mjs confirming that the regular Conext Configuration Tool won't adjust the DC voltage reading calibration. That's the exact info, good to have, when reaching the first line of Schneider support where they might not always "hear" the details of the problem. Could very well have saved my neighbor $600.
When I first did the commissioning, back in April of 2022, just as now, the AC1 outputs were within 0.2VAC, so didn't give the need for the tool much thought. Also seems logical newer Conext Gateway/Insight firmware could handle the AC1 measurement
match calibration, because as Shimmy pointed out,
accuracy to a standard is not as important.
The "engineer" from the online "solar" supplier that did the original spec document did not include a battery monitor. To be honest, after seeing that the Bulk & Absorption cycles complete every day as long as there is enough sunshine, there may well not be a need for one.
I did think about trying to use Modbus to sort of emulate a BMS. But the way I interface to it from the OpenWRT router is Modbus over (WiFi) TCP through the Conext Gateway. Even with a bit of a lag in that network path, setting the Watt power limit every few seconds does not cause a problem. Buuut I think with as tight of a closed loop feedback that the MPPT switching charge controllers need, that voltage info feedback needs to either be right on the same Xanbus/Modbus segment as with a BMS, or be internal to the charge controller.