>why take a risk on a single mother with spotty employment history who is quite likely to fall behind on rent

Yes, it seems unlikely that a market-rate rental in Brookline(!!) is what's being discussed, if all that's true and yet this particular firm had already been working with OP for a month, regardless. "Spotty employment history" but they get denied on their credit so their single-mom income is fine, I guess? for a market-rate rental in Brookline? ????

But if it's not market rate situation then where all the "well-paid professionals" fit in to the story is anyone's guess. Maybe they are a product of your imagination.