As for Grand Master being Alvin, yeah it's true. But about the assassin at the end. I figured something like this: When alvin teleported to somewhere, possibly to another time, I believe that he traveled past his own timeline, creating two Alvins. Okay, since the Alvin from the future who traveled to past, is now alive before he was born, he was taken as a pupil by some witcher, possibly one from one of the other schools, he underwent the training and mutations and became a witcher (I base this to the phrase in which Alvin says that he'd like to become a witcher, altough, in -THAT- time it was not possible, so when it was possible in the past, he took his chance). While the future Alvin was being trained, another Alvin was born. He, instead of becoming a witcher, learned to use his inheritanced powers and later on, became the Grand Master.Then the events of The Witcher take place, and meanwhile Geralt is killing the Grand Master, the younger (born after the first Alvin traveled back in time) is hired to kill the king.So, when Geralt is getting his reward from the king, the younger Alvin enters the castle, and attacks the king. Ofcourse, as seen in the film Geralt stops this.So now you ask, on what I based this. Well if you compare the charesteristics in the Grand Master who is shown in the game and the assassin showed in the engding film, you can see some, in fact, quite many features that look the same. The nose, assassins beard (and hair) color, and stuff like that. And he also kicked Geralt's leg, which had been crushed before, altough this could have been result of his training so he could notice flaws in opponents defence.Altough for this to happen, it would require that the two Alvin's never met, because that would create a paradox.This is my view of the issue, but as we know, it's just a theory.