I have created a custom field that is a "Single Image". I gave it the ID "_thumbnail_id" so that it replaces the native WP featured image. I selected it to "Show as an Admin column" which it does.
Now, how to make that thumbnail linked to the post's edit page ? It is the most obvious thing my client will want to click on (title is secondary).
The subject has already been addressed here but was never given an answer (appart from use another plugin) :