Make sure that the form that opens up the Check Box Selector is triggered by a microflow. Within that microflow create the associations on the referenced data WITHOUT committing into the database, this way the selections will show up populated. If the user selects the save button then the list would be committed, otherwise the a close/cancel button would disregard the associations (if the associations get committed for any reason) you can also add a rollback activity to the close/cancel buttons.
You could add microflow buttons on your form that perform the operations add/remove all. But It would be even better if the creator of the widget would add an optional row and column that if marked marks the whole row or whole column.
Cheers guys. :)