The path of least resistance is to find another joint sponsor. You could write a letter to NVC. However, the chances of that working are unknown. Has the I-864 been accepted or rejected? If NVC schedules an interview, the Consulate Officer will be the sole approval authority for the visa. He/she can either accept the I-864 without an I-864a...or he/she could demand one. As I said, the easiest path is to find another joint sponsor.....or convince the spouse to sign the I-864a.