Why New Ironsides could not hold Warrior

Could New Ironsides hold Warrior while monitors attacked a British convoy?

No; Warrior would not be tied down, Black Prince would reinforce her, and the British could exploit ramming or other coastal tactics.

New Ironsides is too limited to detain Warrior in open water without suffering a decisive response. Black Prince would also be present, and either British ironclad could use ramming against low-freeboard opponents where sea room allowed it. That makes the proposed division of labor fragile because the American broadside ironclad cannot reliably prevent British heavy ships from turning on the monitors or transports. Near shore the tactical problem changes, but Britain would have additional methods and vessels available there as well.