The entire bit about using the shadow variants of spells to bypass casting time has an intuitive solution to keep things from being stupidly broken.
When you cast a contingency spell, the casting time is at least ten minutes. If the spell tied to the contingency's casting time is longer, it takes that long instead.
So any spell you choose to simulate with Shades/Shadow evocation should take one standard action /or/ the casting time of the emulated spell, whichever is longer.
It's the easiest and most reasonable solution.
