I have setup a group using the UI. Inside a have a single_image with the ID _thumbnail_id, it does not show in the admin column, I have read the MB admin columns, does not support displaying anything from a group.
Here my question, is there a way to show the _thumbnail_id on the admin page ? for example wouldn't it be possible to setup another single_image field outside the group (it has to be read_only then) , that will 'hold' the value of the _thumbnail_id field ?
If yes how can I do this in the UI ?
Yes, if you want to use the single_image field to set the featured image of the post, you need to use it as a top field (outside of a group field), then disable the featured image of the post.
For the CPT, you can go to Meta Box > Post Types > Edit the CPT > Support tab > Disable Thumbnail option.