I had a lot of trouble keeping track of the characters. Eventually, i stopped and made a list of who's who to avoid feeling so confused.
Mrs Marchmoor is AKA Margaret Hope and did work at the brothel. She apparently disappeared about 4 days before, according to the brothel owner. I think that because she is the daughter of a ruined mill owner, she may stand more chance of fitting in with society than maybe some other of the women. I can't remember anything about Miss Poole's back story at all. My guess is, though, that she's very good at hanging on to the coattails of people. She enjoys having power over others and is sycophantic, so may have made herself acceptable to the group because of those qualities. Angelique is both very professional as a prostitute and discreet, as well as being less likely to be missed, having grown up in an orphanage. Except of course by Chang. Angelique is there more as an after thought because of what happened to her but I think it's significant that there are 3 of them - to offset the 3 heroes and because of the religious element going on in the story.
I think the prostiuties are chosen because of their professional expertise and insider knowledge about society. But I also think it may be relevant because they are "fallen women" as they would have been known as in Victorian times and again, it ties in with religious element that's sold to the crowd. They kind of fit in with the whore v madonna female stereotypes. And if prostitutes can become angels, anyone can.
"Luck... inspiration... both only really happen to you when you empty your heart of ambition, purpose, and plan; when you give yourself, completely, to the golden, fate-filled moment."