The AD736 is just a place to start without a schematic Probably because of the TRMS capability, the attenuator has to be designed differently. The AD736 takes an AC signal and converts it into an RMS DC value, so it's a good place to start when only the AC measurement doesn't work.
Unless the OP amps are acting as comparators, looking at the difference between the inverting and non-inverting inputs would be something else I would look at. Check to see if the outputs are at a rail. The difference should be less than a few mV.
We don't know what the analog switch does, but you could look create a table of the logic inputs and measuring mode, to see if their is correlation. It MIGHT route the converted AC signal to the DVM and the other signals. The 4066 is a relatively fragile device, if the supply rails are exceeded. Then you can compare the inputs and outputs using the truth table definitions. If the state changes are dynamic, then it becomes more difficult.
Nothing is certain. You can only hope to find something wrong. Once you have one working, it can be come the basis to fix the others.
One heard part is finding the "reference". The power pin for the AD736 looks like a good choice. you might try to figure out if this is connected to meter COMMON with an ohmmeter.