Alvin being the Grand Master sounds logical enough, the assassin at the end, Geralt is simply surprised because of the yellow eyes, I don't think he recognised him and he king also seems surprised as well, he is definately a witcher, and definately not from Kaer Mohren, none of the witchers we see in the game uses daggers, and they are definately daggers and not swords.Another thing no one mentionned, the assassin also carries a potion utility belt like every witcher we've seen in the game, so it would be logical he belongs to a completely different organisation that has little to do with the actual events in the current game. Most likely it will be related to an expansion or a sequel and the organisation/assassin wanted the king dead and saw an opportunity to kill him after all the chaos and troubles and death that had happened in the city (knowing the guards would be weakened, and there also would be a lot less of them.) They might even have wanted to do it while Geralt was in the castle and try to put the blame on him, assuming most people don't know about any other witcher school other than Kaer Morhen. And since Geralt was a public face in Vizima (possibly the only witcher those people knew since their numbers are so few) putting the blame on a witcher would invariably lead to Geralt.Another thing is, did anyone think this ninja/assassin looked like an elf. When I first saw him at the start of the cutscene I was sure it was an elf sent by Yaevinn or maybe Yaevinn himself, now we know that can't be true since he has some beard, but what about a Nilfgaardian assassin with witcher mutation, the Glossary states that the southern kingdom of Nilfgaard is the one where there is the most mixed blood between elves and humans, and since they were just at war with Temeria 5 years before the events in the game, they would have very good reason to want to take this opportunity to assassinate King Foltest after his capital was just ravaged and burned down by a failed coup d'etat by Alvin/Grandmaster.