I really didn't base anything on the Denarian Trilogy, I could of sworn it was mentioned in TDF, in Death Mask I think that the Fallen Angels were exiled, and were in a similar place to the Outsiders, in complete and empty darkness or something like that. And they use the coins as the only manner to exert their will on Earth. I don't remember anything about the coins being destroyed, freeing the fallen. If not I would think the Denarian's would stop whatever their doing and set their number one priority in destroying their own coin.

Edit:

Never mind I checked the books and I couldn't find anything, but where does it talk about destroying the coins, and freeing the fallen inside it?

 

In a post by Jim Butcher on his forums. I'll go find it later.

 

Lash's comment about the "complete and empty darkness or something like that" is the sensory deprivation of being trapped inside a coin, not from being Outside. The Fallen in the coins need a mortal conduit to experience the world, as Lash says in PG, literally, as all its perceptions can come only through its hosts.

However, while flicking through the books, I'm also not finding the bit where Lash is actually angsting about the void either.

 

Maybe it would be like making a horcrux? To make your soul whole again, you must feel remorse for what you've done, and for someone who's soul/being is so mutilated, the act of remorse/repentance would cause extreme pain.

The denarians are pretty high up on the food chain, but it's been proven that they are no match for straight up divine power. This holding true, the fallen are not as powerful as they were when they were God's angels. This step down from their power leaves their being damaged, and to make it whole again they must repent, and bear the extreme pain?

I don't know. I'm a Christian, and I've gone through a lot of scripture, but I'm pretty sure that the Bible is God's message to humanity. I have absolutely no idea what He does in regards to his angels.