I believe you are correct. Basically in order for either Disney or Universal to explicitly use the Black Panther character they would have to go to the negotiation table. However, Disney is very likely free to use Wakanda and most (if not all) other characters without breaching the existing contract.1. Black Panther is off limits for WDW, but it's a question where "Wakanda" or any supporting characters from him would be off limits. The publicly available contract does not really talk about such details.
2. Technically, I don't think Universe has rights to use Black Panther either. the contact basically says that they lose rights to additional characters if they didn't use them in a (long since passed) time frame. Black Panther is prohibited from other parks east of the Mississippi not because Uni is using him and has the rights, but because he is in the "family" of other characters that are used/rights owned by Uni (e.g. Captain America and the Avengers family).
I believe the hiccup is two fold. Black Panther exists as part of the legacy Avengers, therefore he is covered by that aspect of the contract. He is also represented on a mural currently in place at Marvel Super Hero Island.