I agree, I read and re-read the Grasping Shadow and Phantom Chasm powers and I don't find them ambiguous, nor do I find any real loopholes in how to exploit them, and while it may seem inconsistent, I definitely see reasons why Rodney Thompson designed the powers in this fashion.
As Mort Q already pointed out, the slightly longer immobilization time inflicted by a miss with Phantom Chasm is the way Daily powers without the Reliable keyword are structured; they still provide a relatively good effect, though not as good as immoblization, the penalties inflicted by being prone, and 2d6+Int modifier damage. Likewise, for grasping shadows, it is easier to track durations expressed in turns for a later event by starting at the time of that event, not an earlier one. Also, you can't really being your turn within the area of effect unless you were targeted by the attack, so you would already be covered by the first part of the power.
As for Conjuration, I definitely see why these powers were not marked with that keyword (or Zone for some of the others). Conjuration and Zone powers actually conjure real objects or creatures into a space, while Illusions are just figments, even if they seem to occupy a square. The ability to interact with them seems irrelevant.