A lot of detail is lacking.
How is a user changing data in the supporting scenario if they have certified? If they have certified, they should no longer be able to get at the Form to enter more supporting data without Unlocking/Uncertifying. I'm presuming they are in turn running a Custom Calculate package to copy the data from the supporting Scenario to main Fcst Scenario. If so, I would also include that package as one of steps that also gets locked when they certify. That way, WF is doing all the work for you.
