From the way Corb talked I'd more suspect her to be Nimue? Morganna wasn't a mewling child in Arthur's day, Nimue tho.. an I don't think even Merlin would have authority to cast out the kings sister. I took it more under the context of dismissing your apprentice, and as the court mage casting her out of the court.
I know traditionally Merlin had the romance with Nimue...but as a Fae, Nimue was old when Merlin (OG) was young. Unless Jim is rewriting Nimue to have been human originally at the time...
Now, a mortal witch who was a "mewling brat" around the time of William the Conqueror? That could well have been Morganna, depending on how Jim is incorporating Arthurian myth into history. There are indeed many stories of Morganna and Merlin being connected, both as master and apprentice, but in some cases lovers too.
As for Merlin casting out Morganna - he was
Merlin. He built Demonreach, created the White Council, reformed the Vatican, rescued important knowledge from the Library of Alexandria when it fell, and jumped over tall buildings in a single bound. The guy did what he liked. Who could really stop him? If you read the stories, he essentially comes and goes as he pleases and tells Arthur (even as King) what to do all the time. He is an adviser, but he doesn't really see himself as a subject. And Arthur
highly respected Merlin (as did all the Knights). If Merlin said Morganna was to be exiled, I think they would hurling her out the door before he finished his sentence. Not to mention, consider why Morganna was exiled? Likely dark magic - which is even hinted at much in the old tales, and I can't see Arthur giving his half-sister a pass for that. Arthur wasn't Michael Carpenter in nature - just read about how he takes Guinevere cheating on him. Guy could be pretty wrathful.
Just for argument sake, a couple of points;
1] The prisoner has been in that cave for a very long time.. It is very possible that he learned modern English form the various Wardens. Though admittedly why he'd have a British accent is a mystery.
2] Corb might not know that Merlin is in the cave on the island, unless told, he wouldn't know.
I get what you mean, but Jim was asked about the BP being Merlin - and the answer he gave was that it wasn't very likely given Merlin would still be speaking old Aenglish/Welsh. Whoever that being is on the island, I wouldn't necessarily consider him a good guy particularly given he was much further in than Naagloshii.
I mean, he might not. But given Mab's reaction to Corb's words...I'd say she thinks he is dead too. And I'd say it's less likely she wouldn't know that Merlin is on the Island, if that's were he was. Beyond that though, most of the supernatural community believe Merlin (OG) is dead. Hence the entire need for a replacement one on the White Council. Which isn't to say that he is dead (or completely gone beyond the veil), necessarily speaking.