... Namshiel is only able to operate in the mortal world because of his hosts free will, and Marcone is no wreck like Rasmussen.
Note that the Denarians
cheat. Acting to counter that cheating is one of the things that gives Uriel & Co more scope & agency, themselves.
Also note that a malign Angel could swiftly break ANY mortal will, even Marcone's. That the Fallen do
not do that is (I think) one of the limits that they choose to abide by, specifically to prevent non-Fallen angels from acting to oppose them.
I expect that -- in general -- each Host makes their own separate "Deal with the Devil," specific to the Host/Denarian pair. And Denarians are good at negotiating those deals;
hellishly good (outclassing even Marcone).
I think the Coins themselves have inherent "deals" -- the deals whereby Fallen Angels get to exist within the mortal realm --
no summoning needed!!?! oh em gee ! -- for just anyone to pick up.
But then, within the limits of the Coins and of the Host's deals, the Denarians have a fair bit of leeway; in fact, everything not explicitly forbidden is allowed.