For even numbers of projectiles, the logic could be to adjust 10 degrees in either direction and fire both. So, a fireball would become less accurate due to the spread at long range. With an odd number, the center would be on target, with the others spreading around it.
In the case of Ice Cross, you would rotate the cross slightly. With 2 projectiles, you would get, in effect, two long, skinny X's. The Fire Nova spell would get more coverage.
Meteor shower would be awesome.
I don't know what to do about shields. But, as an enchant is doesn't have to be good for all cases. I only ever use acid gills underwater. This could be a player decided situational enchant instead of environmental.