I solved this by calling in, with my wife next to me, and telling them she wants me to be able to use her credit (and this was not even me traveling with her -- I wanted flexibility to use it as we live in China and with COVID and her getting pregnant, ZERO chance she could use her Delta e-credit).

They made a note in the record and I was able to use it. But I had to call in to make the reservation and have that e-credit applied. Didn't work online like they told me it would.