Those are all good points, but I think there’s a difference between reprieves and pardons. The constitution says he has both powers. The pardon takes away the vulnerability to prosecution by altogether taking away the crime. The reprieve — when reprieve is defined as relief — means that he can grant them relief. What relief can he grant them that would best fit his agenda?
Work status.
And, as you have so carefully pointed out in the past, once status is granted, some court will find it a violation of our laws to have a group that is in some kind of limbo status — allowed to be in our country but now allowed to file citizenship paperwork. Allowed to work, but not to participate. Some court will find that the equivalent of servitude.
I do not believe Obama has the Constitutional authority to grant anyone a specific right to work absent them obtaining a valid green card under the existing laws. Now they could become eligible to apply if somehow their status was changed from illegal alien to invited guest. But I don’t really see that as implied in the pardon clause. Only Congress has the power to change the laws and actually grant a wholesale change in status.
If Obama tries it, and I am certain he will, it will be challenged in court. What happens there is anybody’s guess.