I would love to, and in general I do, but I do all my development in a VM, and my Neon VM doesn't expose a brightness setting, which is the control that inappropriately appears on top without this change. So the before-and-after wouldn't be very exciting. :) If anyone else wants to give it a whirl and post a screenshot, I would be very grateful.

I usually just temporarily mess with the dataengine to pretend to have brightness controls for testing :)

To be quite honest I don't think there is a complete agreement on the layouts of the systray dialogues, although one should be considered - but since systray dialogues have a bit of a history and there are still plans going about another systray dialogue layout, we should be careful with adding work to current design.

ON THE OTHER HAND:
As long as the edits we plan are sort of careful we could create a unified ideal for the system tray popups with an eye towards eventual future edits. Is there an interest from a developer POV to create such a thing?

Say FOR EXAMPLE (an example, not a clear suggestion or goal) - we could design all popups to follow a simple layout like this https://imgur.com/a/xbGt1

Let me explain the situation a bit: HIG for the systray content is in progress (and no, it won't be a complete redesign, but just a formalization of the best charateristic of the current plasmoids) so some modules *might* have to be partly reorganized afterwards.
an initial series of mockups is due soon on phabricator, so a discussion similar to the one on kcms can be done.

as for the immediate problem, moving the checkbox now may be good, if improves things as is a very small change. (it just may be necessary in the future to move again or make further changes even tough minor)

That's fine. I was just aiming for a small improvement here and to get a bug closed.