It did surprise me to learn that not only did the Genoskwa survive, but was able to actually leave Hades' Underworld.
Then again, Denarians have shown high levels of recovery in some circumstances, and Ursiel is about as tough a Fallen as they come. Coupled with the Genoskwa's own toughness (and likely ability to heal), plus whatever other Denarian tricks (like speeding up the mind) I can see how Blood-on-his-Soul survived.
The only way I can see Ursiel and BohS escaping is through a deal with Hades himself, either negotiated directly or perhaps Hell (as in Lucifer's domain) intervened. Maybe a trade, or even a jailbreak (although the latter seems less likely).
As Morris points out, there are essentially only a few ways out i.e. the main gate guarded by Cerberus (and you have to cross the River Styx, pay the ferryman etc). Or the backdoor entrance that Dresden had opened, which is harder (if not impossible) to open from the inside. I don't know if either seems likely.
Of course, the Black Council might have had something to do with it. They seem to have all kinds of tricks up their sleeves.
To answer the other OP questions:
1) I doubt Ursiel will care that they were left behind, but the Genoskwa is fairly emotional and might be upset. However, the source of their rage will of course be Dresden so he will receive the brunt of it.
2) Any mortal is corruptible i.e. it doesn't matter the species or race provided they have a soul and free will. Most don't have one let alone both. Therefore, there are many mortal races beyond just humans that could also take up coins. For instance, a White Court Vampire (this scenario has been put to Jim and he said the Fallen would easily outmatch the Hunger, no contest really). Changellings, some Scions (i.e. half-mortal half-supernatural creatures), Forrest People, Wolf People (like in the comic), Tera West's species (wolves that can turn into humans), etc. There are probably many I have left out or forgotten. It's the mortality thing that's key. I will say most animals probably are not susceptible (otherwise Ursiel might just pick a bear), probably because while mortal most (if not all) do not have souls or free will.
