The Conniver - Maybe this guy can be the one with secret ties to Sabrian Melchazier (though the Conniver thinks its' only Philip Dusaine). He's made an deal with Sabrian to get the funds to do this with. Wheels within wheels.... this could get complicated.
With regard to Sebastiano - I was reluctant to make him part fighter, since he should be able to handle pretty intensive magical duels. However, if he's well-built, and has several well-chosen wands, he could probably be as useful in this role as a high-level wizard. I'm not really sure what the most effective way to combat rogue spellcasters would be - without using prestige classes, or just making him a monk.
I can do the build for Oakheart if you would like, not much of a min-maxer, but I'd probably pump up his unarmed stuff ok. He's probably very well-trusted by the other mages as well - they've all given him their books.
Lastly, I think Cor Mahael should fit outside the demographics. He's an old elf, and lived in an uninhabited, swampy, ruin (before Mor's End) in a great big tower. I don't think he really counts as a 'city-derived' character, and so its safe to make him a little more powerful than most mages in the city. With that said, I think the scaled levels are a very good idea - he can be very powerful if the PC's start off high-level, or just be a handy wizard (9th is probably more than handy I guess) to leave the PC's work at low levels.
(P.S.)I think we should refer to the Wizard's Guild by something else - Arcane or Mage replacing the term wizard. The way it is now, the sorcerers are actually as active a the wizards, making it sort of odd to call it the Wizard's Guild.
(Edit - few little errors.)