When merging: require row to be taken across before changing row settings

When manually merging, you can change the 'settings' of a row before you have selected it to be taken across.

Users could miss the fact that the middle column ('merge this data') is not checked, so the value will not be taken across, and their 'transformation' not applied.

See screenshot (it explains it better...)

The proposed solution is to disable the form options for 'add new' 'set primary' etc until the middle checkbox has been selected. Ie: don't allow any settings to be selected until you've chosen to take the information across.