Joakim, hell no. I don't think we should be able to control it, or that it should have the effects you describe. The blessings of gods would be a semi-permanent low-key affair, while magi as we would think of them could just be told to "do magic" at a certain spot. The effect, the duration etc would all be fairly random, so if you wanted big effects you would have to gather lots of magi and tell them all to "do magic" in one spot (simulating ritual magic) to ensure that there's always something going on.
We can control what material our swords are made out of, how each and every piece of furniture in our fortress is placed and exactly which cow lives and which dies. So there is some precedence when it comes to fine control. Still that level of control is a bit too much for magic, I agree. But putting some guys in a room and just waiting for them to come up with something doesn't feel very magical either. Magic should be about stubbornly spending years and years struggling in a tower or dungeon searching for the answers of mysticism.
Rituals should have to be researched. While searching for the solution the magi will need all kinds of peculiar ingredients, live specimens and books which will probably be in when it's magic implementation time. All randomized of course. And you do not know what he will come up with, or have only a rough idea. A permanent mood if you will.
And when it comes to effects, raising of the undead is a powergoal. So replacing maybe the regenerated champion with resurrecting a champion as a death knight seems reasonable.
Demon summoning is in the Moclem story, so that's in too. Or not definitely out anyway.
Another thing to consider is summoning demons to make deals with them, like in the "Summoned to Darkness" story. In that story it doesn't seem to be that hard to summon a demon just to have a chat. Though we do not know how long he had to search for the blasphemies to perform. A deal can either be a "When I do X I get Y", like in the story. Or "I really need Y *now*, I'll promise you'll get X". Then you'll be indebted to the underworld, which can be an interesting cause of Fun.
Actually I think various kinds of summoning and godly intervention only would make for a good magic system. There would be an intelligence behind every spell, which is an excellent reason why things might work sometimes, sometimes not, sometimes once, etc.