Which Level will HE observe?

The Level is set in BOTH Set color temperature per mode for these bulbs

AND

Set these dimmers per mode.

If you don't set the Dimmers you get

So I have set the dimmers at competing levels - which one will HE actually do and how does it decide on complicating settings?

Initial testing appears that whatever you put into the mandatory Dimmer field is ignored if the Color/Level is set, which begs the question -

Anyone know why getting Dimmer Level is mandatory if ignored?

Just see a hint

It appears to go to the Dimmer Level setting for a fraction of a second then goes the Color/level setting.

These are two separate actions/groups of devices, and my assumption (untested, but seems consistent with what you've said in your most recent post) is that you're seeing the level get set first, then the color temperature plus level, effectively resulting in only the CT and level being set to the second set of options when all is done (assuming all the devices respond appropriately, which some won't if they're flooded with commands this fast).

Why not un-select "Set these dimmers per mode" and just use "Set color temperature per mode for these bulbs"? Since you can set level (the attribute that "dimmers per mode" is also modifying) with the color temperature, you don't need the "dimmers per mode" option at all. (The key, again, is to un-select the devices in the "set these dimmers per mode" option; once you've selected devices, the rest is required, so this is how you avoid the message in your second screenshot. If you have additional devices you'd like to set that are just dimmers and not CT or RGBW bulbs, you can choose just those specific devices there instead. Otherwise, just leave it blank.)

If I deselect Dimmers I get an error requiring me to complete it.

You mean even if you un-select all devices? (Not just the level settings; this is different from what your second screenshot pictures.) I am able to save this just fine--notice, no devices selected for "Set these dimmers per mode":

image

If that doesn't work, I might try re-creating this app or using a different browser to see if it works differently for you then.

You are quite right, disable all devices from the Dimmers and that allows a save.

Thanks for that!