Re president's payroll tax deferral: If employee earns $5K in a bi-weekly pay period ($1K over the cap) - do we defer withholding on a prorated basis? Or do we defer 0?

Zero. Anyone earning an annual pay rate of nominally $100k but more precisely $4k per two weeks is ineligible to have their employee ss deferred.

That is, if you are going to participate in this fully VOLUNTARY AND NOT REQUIRED program.  Remember THIS and remember it WELL>>>>>>You , as the EMPLOYER, are ultimately responsible for paying this deferred tax sometime before 4/31/21, yes YOU.