I think the idea that Demonreach can't simply lock up the coins without a bearer is laughable. The church can't hold the coins because it puts the coins in the hands of free-willed humans. It's like hiding a steak in a dog kennel. Demonreach is not a free-willed mortal. He imprisons things that are WAY more powerful than some pathetic ex-angels, and can't be influenced by puny nickleheads that can't even manifest outside their coins.
The Denarians are "locked" into their coins because of a deal. It gives them a solid presence in the world, interaction with mortals, that they couldn't get any other way. It lets them set long-term plans into motion, guide and nourish those plans, revise them at need, take advantage of new/unexpected opportunities, etc. All the other Fallen are locked out (except for all-too-rare-for-their-tastes-summonings).
Other than the limits they face of being coin-bound without a mortal host, I believe the Fallen Angels are still, roughly, at the same power-level they were before their Fall (though I don't think that's ever been made explicit in the stories or any WoJ?). They may have
Fallen, but they are still
Angels. The sheer potency of that is beyond all the Earth-centric beings we meet, even the ultra-powerful ones.
In the Dresden'verse, Angelic-level beings are more powerful than any others we see; Uriel, in particular, is known to have the kind of power that could destroy an entire galaxy without even working at it -- and he's more a "
subtle" than a
mighty type! Mab and Vadderung and Ferrovax are orders of magnitude weaker.