None of the classes I have are unable to be grandmasters in alchemy but at least I can buy the potions. :)
Recruit someone higher level (and of the appropriate class) and invest all their spent points in Alchemy. Then if need be take them around to the promotion quest givers and I think they can get experience for those quests even after they are done (note, memory here is really dodgy, I may be crazy). Level them until you can get GM alchemy, and just leave them stashed in the Adventurers Guild in Ravenshore until you need some potions made. It's not a smooth solution to the problem but this can work. Particularly when discussing some of the very high level characters.