No not impossible that she's telling the truth, just a bit unlikely.  And generally, Butcher hasn't had much in the way of unlikely things happen.
I mean, to save up money like that & never get a decent place to live, seems odd.  To me it seems like dream job & just walking away seems like a big deal.  Yet Marci did it without any second thought?
Then 2 characters act like there might be something up with her?  Another red flag for me.
All that means for me, is that I intend to keep an eye on her.  She might be legit, bit I don't think so.  As for who's pulling her strings, no idea.