In reply to Garry HJ.

But can it do so in a “nice” layout of options with graphical pre-decision visual clues, tips, and selection grids in the way that a full width choices/options/config page can?

This is a separate question/issue, and should be treated as such. As for graphical, pre-decision visual cues: yes. You have a live preview of your entire site. Might use of the Customizer necessitate some changes in the way that a given Theme's options are organized and presented? Sure.

Yes, we’re back to the earlier argument about a narrow sidebar being unable to house all the choices and supporting user-help that the current system can offer – you still have not satisfactorily addressed that issue.

The Customizer is not limited to a narrow sidebar. The issue is already addressed, in multiple ways: use CSS to modify the appearance of the sidebar; use a fly-out panel the way that Widgets are integrated into the Customizer; use a dialogue the way that the core media manager is integrated into the Customizer. Or try something else. The Customizer API is extensible.