I can answer some of these questions based on my research.
1. ADGI wanted to improve upon some of the plot inconsistencies in KQII-III. Although KQI was an obvious fetch quest, KQII's storyline was very sparse and there was a lot of things never explained, even in the background material. ADGI wanted to improve upon the storyline and help try to provide a more cohesive overall storyline of the series.
2. Blackthorne and Klytos (the heads of Infamous Adventures) stated that King's Quest III was one of their favorite King's Quest games, and so for the first game that they decided to remake, they thought that King's Quest III would be the best choice. The IA team was still fairly inexperienced, as KQIII was the first game that they remade. ADGI had also been planning on remaking KQIII and IV in order to bring the games' stories more in line with the story that they had set up with the KQII Remake. They ended up never starting development on a King's Quest IV remake, as they instead went commercial and formed Himalaya Studios.
3. KQIII by IA is closer to the original than the ADGI version; however, they did have added scenes, and a few new characters. You can see some of the stuff here:
http://en.wikipedia.org/wiki/King%27s_Quest_III:_To_Heir_Is_Human_(Infamous_Adventures)
4. The King's Quest I VGA remake is a literal point and click remake of the SCI-0 remake, with no real differences that I am aware of (besides the obvious gameplay/graphical updates).
5. Stika already said this, but TSL is a fan game, and as such, I'm pretty sure The Odd Gentlemen will probably not consider it canon. They may throw a random easter egg or something, but I wouldn't hold my breath. From my understanding, they're only planning on keeping the original developed games canon.
6. I haven't played TSL yet; however, as you mentioned, The Black Cloak Society was something that Jane Jensen came up with, and while it is obviously canon, the only specific people in it are Manannan, Mordack, Shadrack, and Alhazred. Roberta has specifically said that she never intended Hagatha to be connected or related to any of the above. That is something the Peter Spear, author of the King's Quest Companion came up with. In fact, there's a lot of stuff that is "quasi-canon" based off of the Kings' Quest Companion books, such as Tamir and Llewdor sharing the same continent; Daventry and Serenia sharing the same continent, etc. While there's nothing in the games to contradict this, it is never explicitly stated either. To answer your spoiler question, there is nothing mentioned in the games about that character's connection to the organization that you stated. I will say that the "Silver Cloak Society" is not mentioned in any of the King's Quest games, to my knowledge.
7. Mask of Eternity is considered canon by Roberta herself. I'm betting that it will at least get a reference in the new King's Quest game, but I'm not sure how much it will be referenced. The game itself, while being an obvious departure from the original direction of the series, is still technically considered to be an adventure (albeit an action-adventure), and it definitely was the direction that Roberta felt the series should be going in that time. Not sure as to how much of canon it is considered by TSL, but since TSL is technically not considered canon, it doesn't matter in the grand scheme of the series.
8. The game is set far into the future, old enough that Graham would have a granddaughter to be telling stories to, and who would be able to sit there and enjoy them. Neither King's Quest VII nor MOE have any specific dates associated with them, so it's difficult to place them in a coherent timeline. You could make the argument that they could even happen at the same time, which would explain why you don't see Valanice and Rosella in Mask of Eternity, although this is pure conjecture. I'm guessing that the new KQ game will be set after both KQVII and MOE, though.
9. Perhaps that child is merely the oldest and the one who wants to hear Graham's stories. If Rosella did end up with Edgar, then she and he are probably off in Etheria anyways, and I'm not sure how well travel works between the two worlds. Maybe Gwendolyn is visiting Grandpa in Daventry because she's the oldest? Again, we won't know too much until we get closer to the release of the game.
10. This is a good question. Edgar is obvious Prince of Etheria, but at the same time, fairies in fiction can sometimes have much longer lifespans than normal humans, so it's possible that Oberon and Titania could live and rule over Etheria for many more years, thus freeing up Edgar and Rosella to rule Daventry. Again, the games themselves do not really give us any particular details about who will end up the ruler of Daventry. Perhaps a Knight like good Sir Graham back in the original King's Quest I will end up taking over? This may be something that is addressed in the new game.
Stika- you had mentioned that the new game is supposed to be a re-imaging/reboot? I wasn't aware of that. From my understanding and conversations with The Odd Gentlemen, I was under the impression that they were planning on keeping the lore of the original King's Quest games intact. I was under the impression that this is more like a revival (like the 2005 Doctor Who show).
Hope I addressed all of your questions. If I got any of my wires crossed and made a mistake, I apologize in advance.
Talk to you later!
JDHJANUS
Josh
