Given the priority of this game is to ensure that every character has every possible bit of content, in-character behavior be damned, when Mercy finally gets her cell content finished you'll definitely be able to leash her and she'll just be a bit of a brat about it.
As for extending the leash sequences to forcing the girls to fuck strangers, I'd be all for it (except that it'd be a massive amount of work to add such scenes for every character). After all, just imagine the absolute freak-out that the anti-NTR people who are already deeply comfortable with this game for being a brothel management sim would have. It'd be worth the price of admission all on its own.