@johnrellis
Thanks for the check.
I am not back in front of my catalog and will try a few possibilities.
A third value works, and solves the issue in the metadata panel but then I get an extra entry in the search 🧐
The idea was to have a "Catalog Flag".
By doing
{ value = "", title = "No" },
What I get in the LrC search is actually the "No Value", which is what I want.
This convoluted method allow me to kinda remove the "Custom Metadata" with the advantage of returning an empty field in the Panel
So I can do : "No" form the drop down menu > photo appears as "No" > I change photo > come back to previous = Metadata field is empty.

In the filter that photo has "No value"
Mighty slick for the UI.
But no automatic < mixed > state like this.
.
@johnrellis
More problems.
I set the second value to nil
In the filter all photos that don't have "value 1" appear with the title associated to that nil e.g. "No"
In the metadata panel they also appears as "No"
• In the Metadata panel I selected 2 photos, one with value 1 and one with nil
At this point is not possible to set all selected images to nil via the Metadata Panel because LrC thinks they already are at nil.
My solution: use "" instead of nil.
Metadata Panel will NOT show < mixed > state but at elast it will allow to set all images selected to ""