Ok, I figured it out.
The 3 rectifier bridges: Those are for regular 24 VAC signals like broken needle detector, door open, etc
The control board is only designed for a single feeder bus.
The 470 ohm is the 24 VAC supply.
The detection is done by the ST BTA16 triac.
It triggers on the voltage drop across the 470 ohm.You know the good thing about these forums? It gives you a lot of chances to be wrong!

So, the big triac, the 47 ohm, the two zeners and the big yellow thermistor are there to trigger and provide the full 24 VAC to all the light bulbs.
It is an independent, self-contained sub-circuit that bypasses the 470 ohm.
Since it triggers late in the AC cycle there will be some spikes across the 470 ohm resistor if a light bulb is on.
It's the TO-92 transistors that are further along that do the actual detection.
There's no reason that the bulbs should dim if 4 or 5 of them are on.
The triac/thermistor bypass could be broken.