Finished it… (Spoilers abound!)

Grandmaster = Alvin ? -> I think so.
Attack on the king -> maybe another "forgotten" witcher school ?

Great open ending - I think there will be an addon/sequel, because the map of my collectors edition has a backside ("The Empire of Nilfgaard")

My ending:
Friend of Shani
Defeated The Hunt of the Wild and the Grandmaster
Friend of Elves and Dwarfes
King of Dice Poker
Knight of the Lady of the Lake
Best boxer of the world
Wearer of Raven's armor
Rescued Princess Adda and Abigail

Grandmaster = Alvin -- yep, that was pretty heavily hinted. There was the pendant, and he said something that sounded a lot like something I told Alvin in a tense moment. Pity, that. Poor Alvin…

Attack on the King -- could be, but I don't think so. He was clearly a mutant and carried potions, but his fighting technique was very different -- dual-wielding two short swords rather than using a two-hander. He also didn't use any Signs. IOW, looked much like the mutant assassins in Chapter V, only not quite so mutated.

I was neutral, and a friend of Triss ("just friend" though). Does "friend of the night wraith" count?

Night Wraith ??? - is this the nice Vampire Lady of the brothel or someone else ? - I have played this game in german, unfortunately many characters have different names

Nope, she's Celina the sister of the Noonday Bride. Perhaps you didn't get/do that quest? If not, pity, because IMO it's the best side quest in the game.

I only helped the bride by restoring the mirror and singing(?) something :-) I could not help her sister because I chosed to get my own house from the druid. -> something for the second run.

Ending up having taught the arch villain his moral values like that was definitely a unique twist, not anything I've seen before.

I had to kill her in the Swamp Cemetary

Hard to be cursed out for being neutral and trying to stay out of everyone else's madness, but the points about death following in your wake were pretty indisputable. And the assassin is definitely a better cliffhanger than having the roof fall in on you
a la NWN2

I assume the appearance of Alvin as the Grandmaster is another ending from the attack on the King by another Witcher?
Since I had heard before about Alvin before I finished the game, so I was surprised not to see him.

(Neutral ending)--Alvin appears briefly on the long fight up the icy slope, as himself. Geralt tells Triss he's just an illusion. Also at one point in this progress upwards,the GM makes a few remarks to Geralt that seem to reflect things he said to Alvin during the game. Later, after the GM is killed, he has Alvin's amulet clenched in his dead hand. But it really is just a series of oblique references, and more of an impression than an actual conclusion--still a bit open to doubt. In my ending, Alvin is never actually confirmed the way you mention.

I also saw Alvin appear and Triss saying it was an illusion, and also the amulet in the GMs hands at the end. Though I think Geralt mentioned that the Amulet seems identical to the one he gave Alvin, but that it seemed more worn.
I never really thought that it would mean Alvin would be the GM.
What I heard was a bit like you say, that some thought Alvin was the GM because the GM made some remarks that some thought Alvin also had made.
I just thought it was going on in the end cutscene and not as part of the game.

EDIT: And I actually thought that Alvin would be a new GM and not the GM you kill. Because, I just heard people talking about a GM they thought was Alvin. And I thought it was the end cutscene they were talking about, where a new GM was shown, and that people thought it was Alvin because of some of the things he was saying.

The likelihood really is that it's Alvin, only grown up, that you meet as the GM--remember Geralt asking Triss what's happened to Alvin after the end of Act4, and she says "he teleported away. He could have gone anywhere in space or time"

It might be because I didn't connect the remarks the GM made, to something Alvin might have made. Because I still don't see it.
In Act 3, Sigfried mention the GM by name I think, when he tells you that he is about to recieve a medal from him. So that would either mean that Alvin goes from one appearance to another. Or that he for some reasons kills the former GM in act 4 and resumes his place.
Another thing is that I have trouble seeing Alvin like that, expecially after I feel I have tried to influence him in a good way, during our convesations.
But I won't say its not possible. And it is also strange that the end cutscene didn't say anything about what happened to Alvin.
I kind of feel the end cutscene didn't give many answers. What happened to Triss and I, what about Alvin, what about Vezemir (The old witcher that gave you the quest to retrive the mutagen at the start of them game).
Its a good teaser for a sequel, I just wished there would be some more answers from this game.

Alvin probably went back in time and became Jacques in the past, he doesn't go from one appearance to other.

A. Sapkowski's books never mention there being a different witcher school. In fact Kaer Morhen is most probably the only witcher school in the author's intention.

I got choked up when The Lady of the Lake presented Geralt with his "Excalibur". I also got similarly touched upon reading Berengar's letter admitting his cowardice. His actions seemed so "human" to me and I sympathized with him.