Alexander was clearly meant to be an exception to Manannan's usual rule of killing his slave.
Except in the original KQ3 Alexander wasn't 'special' he was just another slave. The manual and narration in KQ3 made that quite clear, as did later material in later manuals and ingame summaries. Its also a big part of one of the games bad endings if you fail to defeat the wizard on time. You are confusing complexity added by TSL as if it was part of the original game, when it's not.
Some of the TSL developers have stated they chosen to take liberties with the original story and ignore original intent for the sake of their story (Alexander's role under Manannan is one of them). Like I said I'm more a fan of purity of original intent (what was actually shown, said or appears in the official games/documentation, etc).
I don't know if you have played the KQ3 remakes, you'll notice those team members chose to stick closer to the original intent (at least on this subject, if though changing alot of other material), in that it's explained that Manannan intends to kill Alexander, and move onto find his next slave. KQ3IA, went as far to say Manannan planned to send the body back to his family to cause them additional misery, and its said in KQ3R, that he was going on his journies in search of his next slave. At one point Manannan mentions he has plans for Alexander on his 18th birthday, which you can deduce by the vials of hair and other details found in Manannan's house. So even those developers went with the original intent, that he wasn't particularly 'special', just one in a long line of slaves which were to be exterminated when they turned eighteen.
I'm also not that big a fan of the;
That's overly complicated and convoluted for my tastes. Way too much 'six degrees of separation'.
There are the little things like the appearance of the crystal tower looks nothing like its appearance in KQ2.
Alexander and Rosella each became monarchs of their own new kingdoms.
Actually Rosella wasn't the new monarch of Etheria, and if she was to be it could be a very long time. Remember the Etherians are demigods, Immortals. Because of that it's very unlikely that Oberan and Titania would ever pass on the monarchy. Edgar would likely have time to rule Daventry for a lifetime before he'd ever have to step into position in Etheria. That would only be the case if something killed his parents.
BTW, there actually is an indirect reference to Rosella in KQ8, if you listen to the context of Ugly Beast's insight on Castle Daventry; she mentions that Connor needs to get inside and "rescue Graham's family". This suggests he's not only Rescuing Valanice, but also Rosella as well. Actually originally at one point during development, apparently they were going to let Connor explore Castle, and he would have found Rosella in stone. But it was decided that aspect was too much for the game, since they apparently didn't have any puzzles planned for the castle beyond what was left in the game.
Actually Roberta hinted at had she continued KQ9, she might have went with a 'love triangle' plot with Edgar and Connor competing for Rosella's affection.
Otherwise, I do agree with Lambonius. It is a full circle story. Actually, what we got was kind of Graham's early years, and rise to Knighthood. A story we never saw in the original game. In KQ1 we got the point where Graham was already a knight. It's telling that Graham and Connor both end up sharing the same title, "Sir Knight" by the end of of MOE (see KQ1SCI for Graham receiving that title by those he helps).
I suppose what is annoying about MOE, was as you learn from the Oracle of the Tree in the game, that Connor was destined to return to Daventry following the restoration of the Mask. But the ending never shows this. According to Mark Seibert they originally had plans to have an extended ending, where apparently Connor would have stood before Graham, and received the Knighthood officially, and been praised for his actions. They were forced to cut that out, when upper management pushed for the game's early release. The game actually had seen a couple of delays at that point, and it was way behind schedule into its 3rd year of development (actually the maxium time of development for the previous games in the series).