Hi Stéphane,
Looks good !

Some feedback anyway 
What about
"Pin as default segment on that View" ?
"Set as default segment on that View" ?
To make it explicit it applies on that View only ?
What about a pin icon instead ?

How do you imagine to offer to Clear that setting once active ?

Sure
In my opinion it should be only applied per View
Like a "make that Segment sticky on that View" dialog in the segment scorecard zone ?

To be discussed with more users ?

I would believe that the issue is mostly to always replace the "All Users" by a given segment, and make it visible in the GA UI that it is being applied or that it is NOT applied.
Not sure there is the need to have multiple segments at the same time

What I like the most:

• code beautifier in GTM
• minimize GTM panel across pages
• sort CD in GA settings variable in GTM
• in GA, change the page Title to be able to select the right Tab when having dozen of them opened
  
  I mostly don't use other ones, and fear bugs in the GUI / side effects of extensions