AIU conundrum

I could be way off base, but have you tried deleting all of the AIU items from the remote that aren't working and then re-entering them ?

If practical, can you re-route the modular phone cables so that rather than TIU -> AIU1 -> AIU2 -> AIU3, you have, say, TIU -> AIU2 -> AIU1 -> AIU3.  The idea is to see if the problem follows AIU1 or if the problem remains with the first AIU in the daisy-chain from the TIU.  Obviously the switch names on the remote menu will be incorrect during the experiment.

The AIU SW relay contacts go directly to the screw-terminals.  So if you're hearing the "click" I'd say it's fair to assume the closure is being made.  If you have a meter, you should be able to probe the screw-terminal contacts and see the meter "jump" for the 1/2 second (and it's indeed exactly 1/2 sec) when you trigger an SW port.

If you used the Z-Stuff controllers as well as the AIU to control switches, you wouldn't need the added resistors.

If  you hear the relays opening & closing, your problem is not in the DCS system, but the wiring.  ?#1 Are you getting power (I assume you're feeding common, with a hot wire serving each switch motor) to the center terminal of all 10 AIU relays?

Considering the layout, it was impractical and inconvenient to use the z-stuff controllers, hence the resistors. I'm of the opinion that it's a hot wire connection but I haven't found it yet. The search continues.

A fixed composition resistor can fail to open (no power passing through) when overheated. test the resistor with a meter just be sure.