I just went through a very similar process. I went back and forth with support for over a year because the account I was using was on an E-Mail that I didn’t have access to, and I forgot the password. My problem was instead that I could not remember the final security question. I managed to miraculously guess it after over a year and got my account back finally.
The thing is, throughout my fairly long E-Mail exchange with support, it seemed to me that It was never made apparent that providing the latest three gold payments was a MANDATORY step, I provided them anyway, but I don’t think they ever asked.
I think you should contact them again, explain the situation fully (maybe not that the account used to belong to a friend, that might be against TOS I’m not sure). Tell them that you used a gift card and so on.
It’s a long shot, because DECA is strict with their shitty policies and rules which make no sense. But good luck regardless my friend, hopefully you managed to resolve the situation.