The Denarian (the fallen angel in the coin, that is) can release the Host for whatever reason, should it choose to. Howver, the Host is only redeemed if it's the Host's choice to give up the coin, which is why the Knights tend to be very reticent about forcing the issue. The fate of the Host's soul is a primary concern for them, and in a sense it's a loss for them each time they are forced to kill a Host to recover a coin.
The plan to capture and trick a Denarian is a highly risky one. In no particular order: First, while casting magics on the Host would likely be well within the realm of reason, I'm less certain that mortal characters of less than plot-device power could cast a spell that would impact a fallen angel. So whether the "experience what the host experiences" spell would even work is doubtful. Second, I don't see why being trussed up in an imprisoned human would be any worse for the fallen angel than having his coin locked up in a vault for the same amount of time. Third, Denarians are mythic-level temptors. I'm sure they would simply see this as an opportunity to tempt the bejeezus out of a bunch of monks. Fourth, you've now invited a being of utter evil past the thresholds of a place dedicated to good, and deserve the corruption of that holy place that is bound to follow. Fifth, you've created a beacon for any other Denarians that might happen by (the Denarians might not necessarily like each other or play well with each other, but imagine the value one Denarian might place on a major favor owed by another Denarian that was freed from imprisonment). Sixth, imagine what a shame it would be if a Monk who was 'hand-feeding' the prisoner were to suddenly feel the impact of a small piece of metal against his hand as it fell off his ward's forehead...
The list goes on.
That said, you might get lucky. It worked for Harry once, so it could work again. But it seems very, very risky...