Jan Jansen for example ranks fairly low, but thanks to him being a mage/thief that will eventually reach level 9 spells
You need 18 int to get 9th level spells. That means he needs two permanent Int increases in order to get there. I think you can get one of those points from the machine, but I'm not sure if you can put the second point on him through the deck.
No, you don't, don't be silly. You need to chug a cheap potion to scribe level 9 spells, and then your intelligence doesn't affect the casting in any way whatsoever. Did you really think Edwin was the only party member in BG2 capable of casting level 9 spells?
Thanks. Now I have an even lower opinion of BG2 and bioware than before. I never even tried abusing Bioware's lack of effort in the second game in actually implementing the rules. I've never even tried abusing this lack of proper implementation of the ruleset in BG2 since it never even occurred to me to waste time testing something that's not even supposed to happen in the first place. I also don't read guides or look up exploits to use. If I ever know about one in any game, it's because I stumbled upon it and I never stumbled on this one.
You see, according to the AD&D rules, INT scores are required for spellcasting. Characters with lower than 18 INT shouldn't even be able to prepare spells of 9th level or higher. This is supposed to mean unless you can find enough permanent/semi-permament bonuses, Edwin is supposed to be the only companion able to cast 9th level spells by default. It's also supposed to mean that given the limited amount of INT bonuses that endure through rests in game, you are supposed to according to the actual AD&D rules only get up to two other companions if using Jan or Aerie (not both) to cast 9th level spells. The only way to get four characters who can cast level 9 spells is supposed to be giving only one bonus each to Imoen, Nalia, and Sarevok with Aerie and Jan left with only level 8 spells.
Here's the table from the 2e player's handbook as presented in the AD&D Core Rules 2.0 CD.