Garion was set up by Polgara and Belgarath as a puppet king.Bear with me. Garion is raised by Polgara on Faldor's farm, and spends most of his childhood doing menial tasks. Now, while this does make sure he never develops a sense of superiority, and it makes him a very sensible person- both valued virtues in a king-, he was also continually kept in the dark by Polgara on nearly every subject, and this makes him totally dependent on her, because as a very sensible person, Garion isn't going to try tackle a problem he has no idea how to handle by himself, he'll just call his Aunt Pol. This gives Polgara the ability to continually interfere in the workings of the Western kingdoms so she can make sure that the events of the Prophecy happen. Once Garion grows up and gets the hang of the job, he isn't dependent on Polgara any more- but she raised him, so mostly, what he does will be what she wants, especially since he loves her and admires her and has put her on a pedestal. Belgarath, in the meantime, is there to serve as another figure for Garion to depend on, while serving the same goals as Polgara. While Polgara and Belgarath do care for him, they care for the Prophecy even more.
The majority of the characters in The Belgariad/The Malloreon were set up as deconstructions of the usual fantasy stock characters.Take Garion, your traditional 'chosen one who is revealed to be so after labouring in obscurity for x years'. He's a badass, a good king and all- but he's also extremely dependent on the people around him, insecure, headstrong, and rarely thinks things through (remember the potential ice age-causing storm in Guardians of the West?). Ce'Nedra, the 'feisty princess who marries the chosen one' character, is a hypocritical bitch who spends most of her time in books as The Load and often delays and slows the main cast down. Polgara and Belgarath, the 'wizard advisors who help the main character and do awesome things a lot' characters, are both extremely manipulative. Belgarath's a drunken, irresponsible lecher and Polgara's a superior, condescending bitch. And that's not even all of them...
Members of the Drasnian intelligence service forfeit their place in any line of succession they may be a part of.After all, someone who's going to be travelling all over the world and is likely to get themselves killed before they can produce an heir is not the ideal choice to inherit any sort of responsibility. And to approach it from the other side, the intelligence service wouldn't want their agents to be tied down by obligations back home. Therefore they are automatically bumped to the bottom of whichever line of succession they're a part of when they enter the Academy. This explains why Silk is said to be "in no danger of ascending the throne" despite the fact that as the only nephew of a childless king (at the time anyway), he should be if not next in line then close to it.
The King of Hell caused "The Accident"It has been established that Demons exist outside of the Necessity's design and influence, and that UL chained the King of Hell in his own realm at the start of creation. It is possible that in the time he was in the Universe, he caused the star responsible for the accident to move to a place "it was not meant to be", thereby being responsible for the series as a whole.