FWIW I had a similar conundrum with Slack. I had set-up my business Slack workspace in college; 4 years after graduation my university changed policies (they used to forward name@edu => name@alumni.edu).

I tried the normal means (support tickets etc) to no avail. The third or fourth time I got someone in account recovery. There was a very formal process for verifying my identity (I'm sure based on the process this happens all the time). Eventually I they helped me recover my account. It probably took a few months on the whole, but once I got the right support rep it was only a week or so.

So my advice would be to submit more tickets. Because they might have a process that not all support agents know about, and some are more helpful than others.