I don't understand why it's taken on faith that anyone would be sent back under this proposal, since we've been practicing catch and release with illegal aliens for years.
What would be the purpose of setting up a whole new system and not holding people to their promises?
To me, it seems to give more force to a program; smoke them out and then watch them. If they want to earn their way here, fine; if not, let them go back with the rest of them.
Of course, two things have to happen: tighten the borders and set the deadline: after a certain date, no more registration; they have to come through legal channels. Then, when you catch people who aren't registered, it won't be that hard to tell who is still coming over the border and back they go.