There's no undo button within the merge tool (too complex to track easily), so if the users (e.g.) deletes a sense then changes their mind, they have to either defer or exit the tool. Some sort of "reset" or "start over" button could be handy.

• Add a "Revert set" button right after the "Defer" button
• Disable the button when the current merge set's state has no changes from its initial state
• Clicking the button pops up a confirmation dialog
• Confirming resets the state for that merge set
• Add all strings to the en localization file