Unsetting a "read-only" custom column attribute still does not allow that custom column to be edited via the item list grid, even though that custom column is editable via the coding panel.
1) Open the project in review mode and remove the custom field from the item list.
2) Re-add the custom field to the item list.
When the item list grid is populated, Summation Pro caches the properties of each field in the grid. If one changes a field to or from being "Read Only", then one needs to refresh that cache somehow, for the browser to recognize the property change. Adding and removing affected column from the grid will refresh the field properties cache.