This distinction might be subtle, but I don't think Lasciel's Shadow was weakened by the prison so much as she became very constrained in what options she had for influencing Harry
If I may lend my support, I'd like to point out that Harry's circle was a conscious act of rejection but as Michael (or was Uriel) pointed out, subconsciously Harry wasn't ready to completly give up the temptation, which is why he kept the coin buried instead of turning it over.
From this, we correctly conclude the shadow could not openly defy Harry's act of free will (sealing the coin), but as Harry subconsciously wanted the coin to be available, the Shadow could make Hellfire available to his subconscious. Then, when Harry subconsciouslly used Hellfire, his subconscious became open to temptation. When Harry consciously used Hellfire, his conscious became open to temptation.
Serack, just to make sure, Cozarkian's post touches on what you mean by constrained, right? Since Id!Harry didn't give up the coin, Lasciel's shadow could offer Hellfire to him. I must admit, there's a certain elegance to it I find quite appealing: Free Will is never really broken, since only those willing to be tempted are offered temptation.
On the other hand, we know that fully giving up a coin does not require a magic circle (since Sanya did it by just letting the coin go). So it's the "conscious act of rejection", the "concrete choice" that cuts off access to the wielder's conscious mind from the Shadow implanted in the wielder's brain, rather than any magic itself (at least, if I understand your theory correctly). A total rejection like Sanya's presumably requires both a concious and subconcious choice. And the choice can be coerced (by say, threatening to kill someone unless they give up the coin; Quintus Cassius' subconcious mind didn't want to die either, after all).
I think it's safe to assume that the offer of temptation isn't always Hellfire. After all, what would a non-wizard want with the stuff? But there's probably always some sort of offer made to either the concious or subconcious mind that opens the door for full-on temptation and illusions.
When you look at it like that, it still sounds elegant. It also sounds slow as molasses. If the Fallen is only allowed to offer some sort of boost and has to wait until after the offer is taken before they're allowed to actually tempt the person, how the heck is their success rate usually so good that they get a person to pick up the coin in a few weeks at most?
On the gripping hand, this does align with the idea that forcing someone to pick up a coin won't work, because even after you are forced to pick the coin, you still have to accept the first offer of power without either the Fallen or the Fallen's shadow being allowed to tempt you. Unless, of course, they cheat.
Serack, would you mind verifying if your concept of Constrained matches with my understanding of it above? How does it jive with the idea that cheating can happen? How does it jive with the idea that Lasciel's shadow took so long (between Id!Harry using Hellfire at the end of BR to Shiela's appearance at the beginning of DB) to cheat? Was there some further constraint I'm missing, or was she biding her time? Because, returning again to the original argument, the question is whether Lasciel didn't cheat earlier beacuse she couldn't, or because she wanted to save her Ace-in-the-Hole for the best time possible.
The point I'm trying to make is Occam's razor: If your theory requires there to be a constraint (Id!Harry using Hellfire) as well as a choice on the Shadow's part to justify why it couldn't cheat earlier, and my theory only requires a choice on the Shadow's part to justify why it didn't cheat earlier, then I believe my theory has the advantage of simplicity going for it.
[Editted to change the sentence "cuts off the Coin from the Shadow" to "cuts off the shadow's access to the wielder's conscious mind" which I think is the argument being made.]
