Just append "optionUndo=true" to the definition of your group sign-off field like other options at the end of your code.
Then, any decider can revoke/undo own decisions by clicking on the related button as shown on the sample below.
If you are a project administrator or a system administrator, then move your mouse over the name to display additional buttons: edit the group sign-off field definition as well as revert all decisions to re-start a decision, loosing all previous votes:
This needs no longer a complex set-up and should make administrator's life much easier.
If you have configured using these functions before, deconstruct that: please edit your global "Rule" and remove all code checking the "internal control field" triggering calls to these functions. Also, the customfield "internal control field" is no longer needed and can be removed/deleted. Finally, please remove all your workflow transitions explicitly used to set the content of the this internal control field.