You can, the whole point is verifying the domain in O365 by modifying the PTR record and CNAME so that the o365 panel will see it is yours.
Support should've been able to remove it, I know I was able to do it on escalation cases when I was on Fast Track.
I know that the "ambassadors" are really next to worthless, and if you do not have premier or a TAM on your account, you are really SOL on O365/M365/Microsoft products, you need to email the ambassadors managers, continually, and then go up the ladder, if needbe, to the product manager of that support team and just keep doing it.
It really sucks.
But the reason you couldn't add your domain is 99% most likely someone added and verified it. There are tools on the backend (most easiest Viewpoint) and then billing (CMAT) that can verify if domain is added.
I kinda wish you could do like google analytics makes you do to validate your URL (add a DNS record)