Isn't there in essence two actions happening with the Shift Enchantment spell? The first being to pull it away from the target and the second to place it as if it where your own spell?
So as the enchantment is being ripped away you could dispel the Shift Enchantment spell.... stopping the the enchantment from leaving the target. If you don't stop it then, it is in essence in the new casters hand, at which point it is cast on the new target, where the Nulify spell then destroys it once it is revealed.
At least that's how I would think of it, anyways.
Also.... who controls the curse anyways? The mage who owns the creature , or the mage who cast the spell in the first place?
The problem with that is that you can only stop the spell during Step 2, and the effects, be it 1 effect or more, don't happen until Step 3.
So even though realistically it may make more sense that the enchantment disappears, the game rules state that all targeting happens at one time, during Step 1 of casting a spell, and you
must reveal Nullify during Step 2, which is before the enchantment starts to move.
I agree that what you are saying makes sense realistically, but that's not how it works according to the rules; and even though AW wants this game to be very realistic, unfortunately for gameplay, it isn't always possible to do that in every situation, without having way too many "what if" rules.