>>73245456I'm pretty sure Niji/Anycolor-as-long-as-it's-black own the IP to the character Selen Tatsuki, I also don't see any realistic way that could be challeneged. I think at most it could be argued that the person who played the role of Selen Tatsuki is entitled to a share of ad revenue from the videos on the channel, but since the videos are not available to the public anymore that's a share of zero.
If Niji and Doki came to some sort of agreement that the old channel were unprivated it would be a divine miracle. It would be one of the best things Niji could do for PR and it could (very slightly) raise their profits. It would resemble something like a good idea and make people think that maybe they are making amends, so naturally there's no chance that they're going to do it.