I finally finished the game last night, for what it's worth these are my opinions, and these opinions were my thoughts as I finished the game - without reading anything here!Alvin and the GM and one and the same, far too many coincidences for me to think otherwise, he was such an important part of the whole story and then just disappears with no further impact on the plot? No, I am certain he became the GM.The final assasin - I wasn't sure about this, I looked at him and thought 'am I supposed to know who it is?' - so I conclude he is just another witcher, I think Geralt was just surprised to see another witcher who he did not know, now whether he is just from another witcher school or is some sort of mutated witcher, I think that will be answered in the sequel (I hope there will be a sequel!!).Great game, really enjoyed it and am sure I will play it again with different choices soon 
edit: I just found this walkthough: http://www.gamebanshee.com/thewitcher/walkthrough/frozenreflections.phpin particular:"After dealing with the King of the Wild Hunt, you’ll finish off the Grand Master by skewering him with your silver sword, which will cause you to re-appear in the Order Cloister in the Temple Quarter. You’ll encounter Dandelion there, as well as the Grand Master’s body. When you loot the remains, you’ll find the Secrets Stolen from Kaer Morhen, as well as a Dimeritium Amulet.Note: Yes, that’s right. When Alvin teleported away at the end of Chapter IV, he actually went back in time, and he eventually became the Grand Master. If you play through the campaign again, you’ll see lots of hints for this, like when you meet up with the Grand Master in the Swamp Cemetery and he says that he knows about you from a “very trustworthy source” -- that is, himself.After looting the Grand Master’s corpse, when you walk over to Dandelion, you’ll automatically start up a conversation with him, and that will conclude the quest as well as the campaign.“Something ends, something begins.”Note: In the ending movie sequence, you’re not supposed to recognize who the assassin is. You’re just supposed to notice that he has witcher eyes. So presumably the Grand Master’s experiments with the witcher mutagens bore fruit -- by somebody, somewhere -- but we’ll have to wait for the sequel to see how it plays out."
edit: I just found this walkthough: http://www.gamebanshee.com/thewitcher/walkthrough/frozenreflections.phpin particular:"After dealing with the King of the Wild Hunt, you’ll finish off the Grand Master by skewering him with your silver sword, which will cause you to re-appear in the Order Cloister in the Temple Quarter. You’ll encounter Dandelion there, as well as the Grand Master’s body. When you loot the remains, you’ll find the Secrets Stolen from Kaer Morhen, as well as a Dimeritium Amulet.Note: Yes, that’s right. When Alvin teleported away at the end of Chapter IV, he actually went back in time, and he eventually became the Grand Master. If you play through the campaign again, you’ll see lots of hints for this, like when you meet up with the Grand Master in the Swamp Cemetery and he says that he knows about you from a “very trustworthy source” -- that is, himself.After looting the Grand Master’s corpse, when you walk over to Dandelion, you’ll automatically start up a conversation with him, and that will conclude the quest as well as the campaign.“Something ends, something begins.”Note: In the ending movie sequence, you’re not supposed to recognize who the assassin is. You’re just supposed to notice that he has witcher eyes. So presumably the Grand Master’s experiments with the witcher mutagens bore fruit -- by somebody, somewhere -- but we’ll have to wait for the sequel to see how it plays out."