His English is perfectly understandable to me, having never heard him speak before and listening on a different tab (so not using the subtitles):
It would actually be nice to hear different combinations of accents onscreen. Adds a little to the more global feel of the stories. Any [non-Namor] guesses as to who he could he playing, assuming he's a human character?
If they're expanding Shuri's role in the story and they won't be trying to CGI Boseman "back to life", I've taken to assume that this means they're just gonna keep T'Challa somewhat out of things for the time being, maybe if he does appear recasted it'll be as a post-credit scene or something from an alternate universe. After WandaVision, Multiverse of Madness and possibly even Ant-Man 3 having aired by then, (as well as whatever Disney+ plus series premiere that might further mess up the multiverse, like Loki), I feel there might be enough craziness for audiences to be more susceptible for an alt-universe T'Challa to more easily get people to stomach the recast. I dunno, we'll definitely see.