Participant wants to defer "income" until retirement. Looking to set up either a funded or unfunded 457f plan. Assuming participant takes contructive receipt after termination of employment are both FICA AND Withholding Taxes due when the risk of forfeiture lapses or just FICA?

I will not attempt to answer your Qs, but I offer the following observation for your consideration. If what you are contemplating is elective deferred compensation by an employee of a tax-exempt entity, I suggest that you give it no further contemplation. See IRS Notice 2007-62.