I just received my Cooper controller (they were backordered for weeks), so I can finally answer these questions.
Unfortunately, even after being associated by the Cooper controller, dim states do not update between switches when they are off. This seems to be a limitation of the Cooper dimmer, and Z-Wave dimmers in general.
I think the problem is that switches don’t obey associations when they are turned on and off via Z-Wave. If you look at the blue indicator on your auxiliary switch when you turn it on from SmartThings, you’ll see that it is indeed turning on — it just isn’t turning anything else on. SmartThings thinks the auxiliary switch is the same as any other switch, so you can use The Big Switch smart app to keep it in sync with the master.
(Btw, if you didn’t already know, you can also use the Minimote to associate master switches together! I use this to turn on/off all of my backyard lights when I turn on/off any of them.)
I’ve spent almost a year testing out dimmers: Lutron Maestro, Lutron Caseta, Leviton Vizia RF+, Wink Relay, Quirky Tapt, GE/Jasco, Enerwave/Linear, and Cooper Aspire RF (Designer and Decorator). At this point, I could probably write a short book about wireless light switches, but suffice it to say that the Coopers won out.
+Easy to use dimming buttons
+Dim level indicator
+Programmable max and min settings
+Rapid start to “kick” on LEDs at low dim levels
+Two-way association between switches
+Instant SmartThings status updates
—On/off button cannot be pressed from the left
—Dim state does not sync between switches that are off
—My LED bulbs hum louder than with other dimmers
Like your wife, I vastly prefer the Decorator style. It’s very similar to the Lutron Maestro, but I wish Cooper figured out how to copy Lutron’s button mechanism. Cooper’s button seems to be hinged on left and pushes in like a door, whereas Lutron’s button pushes straight in without any hinge.
Oh and if you’re wondering about the controller, it’s okay. The screen makes it a lot easier to use than the Minimote, e.g., you can create and delete associations without having to walk up to each switch to identify it, but I don’t think that justifies the cost. The only reason you would buy one is if you want to change your switch’s configuration parameters, e.g. ramp on and ramp off times, max and min brightness, etc.