@Drax: My thoughts are more or less the same about cheating the system; I would argue that's how Ritualists have a variety of spell abilities too which would place them in using Aggression, Preservation, and Destruction magic, but on a weaker scale than to Necromancers, Monks, and Elementalists respectively - for them, spirits are the alternate source.
Exactly. (There's even a little denial mixed in, albeit through offensive spirits that could be viewed as "I summon the spirit of a mesmer!)
When it comes to the professions existing pre-1BE... I suspect they did, kinda, but in a different form. You may have had some people who covered the full spectrum of magic, but you're always going to have some that choose to specialise in a particular area that catches their interest. Some of those are going to be recognisably similar to modern professions - they have the freedom to dabble in magic from the full spectrum, but have focused in largely the same area as a modern mesmer or necromancer or whatever. However, you're also going to have others that definitely do not
fall within the four schools as they currently stand - because they've spread between them, or their area of focus was more where the boundaries of the four schools would be placed. It does look as if the gods tried to line things up with the most common likely areas of specialisation for humans, though - most followers of Lyssa would have specialised in Denial, most followers of Grenth would have focused in Aggression, and most followers of Dwayna in Preservation, with Destruction being what was left over.
Theoretically speaking, this would indicate that somebody outside of Tyria would be able to act without the limitations of the bloodstones (unless the gods decided it was a good idea in general and split magic in some or all areas of the Mists as well). However, that we don't become super-mages in the Mists can probably be explained as the characters using what they know - a mesmer, for instance, might theoretically be able to use elemental magic or necromancy while in the Mists, but it won't help much if they don't know the spells.
(This would make for an interesting variant if the game mechanics can handle it - a region in the Mists where you can use utility skills from other professions. Be hard to balance, though, since the professions are balanced around different strengths of utility skills.)