First of all, numbers really only mean when people joined, they have no business in defining power or relevance (Vexen believes otherwise, though)
Xaldin was number 3, and he wasn't let in on a lot of secrets while Saix, num,ber 7, was, for example.
Speaking of Saix, the dialogues in Days seems to point out Xemnas was fully aware of Marluxia's betrayal, he just didn't care.
Perhaps he always knew he didn't pose a threat and/or Sora would've strike him down. Also, he was probably aware of Saix sending his own spy Axel too (and Axel was definitely the one who crushed the betrayal the most), so there's that.
About the people more deserving... I don't know about that. They either were too important to leave (Xigbar, Saix) or not competent enough (Demyx).
Larxene, not in a million years. Zexion, maybe too young. Vexen definitely too short-sighted for a lot of things, he let Marluxia play him like a complete fool. Lexaeus would make the most sense, perhaps he was too much of a soldier and less of a leader, maybe.
All of this obviously ignoring that by a videogame standard Marluxia was the only character fit to be a final boss.