I don't think the signal path would go through Q209, as the JFET switches usually have the gate at the side with 1 pin. So chances are Q209 is more something for protection of similar.
The input shown on the 1 st. picture still looks a little odd. The large ceramic resistor is not a conventional divider, but some 10 M and than 4 resistors of some 1 M, 100K ,10K , 1K connected to 1 point. From what one can see, this looks more like an inverting amplifier with gain 0.1 , 0.01, 0.001 for the higher ranges. Still not clear on how the lower ranges for 1 V and 100 mV are made. It may be through U404 and U402.
I don't think AC volts has a different ADC, it is more like a different amplifier part and having AC coupling. AC amps may be DC coupled and share some of the amplifier with the DC path. The true RMS conveter should than be common for amp and volts and work OK as the AC volts seem to be OK. It may just be that the current is DC coupled all the way and thus also effected by an offset at the input.
The path for the current measurement may be easier to follow than the voltage part.
Some input circuits have the neg side input terminal different from the normal circuit ground. This can be quite confusing than. So one should check that - likely there is a link, but not always.