Whoa, I just tried that with Shadowheart. Fortunately I had a bunch of scrolls in my inventory, and sure enough, by making her a Wiz 1/Cleric 7 I was able to learn all the mage scrolls up to 4th level spells. So as long as you can find the scrolls, a multiclass wiz(1)/cleric(x) can eventually learn and cast every wizard spell in the game. Wow! I guess the downside is that you can pick only a few wizard spells to prepare, and their difficulty will be based on intelligence ... but they can be of any spell level and not every spell has a saving throw. Huh, this makes me re-think my class progression yet again! The Mists of Unknowing continue to astound me.
