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.
