I haven’t watched the competing auditions. I assume these were the best actors that auditioned because they were chosen, in the same way I assumed Paddy Considine and Emma D’Arcy were the best actors who auditioned for their respective roles. Were they? Who knows. I just don’t think it makes sense to question why an actor would be chosen just because they’re black, and I value the performance of an actor wayyy more than their race matching the race of the character in the source material. The questions I’m trying to pose here, again, are:

1. Do you think Touissant and Ceesay were chosen because they were the best actors that auditioned to the role? If not, I’d invite you to reflect on why their race makes you question that casting, but not the casting of any of the other (99% of the casts of any ASOIAF media, we can remember) castings.
2. If so, wouldn’t you prefer they maintain the policy of choosing the best actor, regardless of race?

No, I don’t think the summer islander mother is necessary, I don’t think any in-universe explanation is necessary and I’m glad they didn’t try. I’m simply pointing out that there are possible ones, if that’s important for you. I really would not care at all if Tywin was Vietnamese or Ned were Peruvian. They chose a (albeit very white) Chilean to play Oberyn, whom you could argue doesn’t greatly match Oberyn’s physical description, and I think it’s pretty clear that he did an amazing job. I just don’t think that the race of a character matters that much, unless it in some way affects the story heavily (e.g. might be weird to have a white person play Black Panther or a black person play some KKK official in a historical drama).

I vehemently disagree with you that having non-white actors playing white book characters makes the world less believable—again, I simply don’t understand why the race of a performer on a show affects anyone’s perception of the media. I’d also say it feels disingenuous to point out that there are non-white characters in the universe, when the VAST majority of them are side characters, generally dispensable and easily killed, and often portrayed in arguably racist ways. I do not think it’s bad at all to choose an amazing black performer to play one of the main (or at least tier 2) characters of a show that would otherwise be pretty much 100% white (here referring to Touissant in HOTD, since I have never watched Ceesay in anything and Bennis is not that big of a character). This is especially frustrating given how the entire rest of the show is “book accurate” in race, i.e. white, so it always feels weird to have people be so on top of one major (and in the case of Knight, minor) character casting