I don't think it's a matter of the plot device so much as the way it was introduced. The discussion between mother and daughter was bit too obvious and openly-avaricious, in my opinion. That could be why it felt unbelievable. If it had been converted to internal monologue or subtle hints instead, I think it might have been better.
I also can't appreciate the plot device called "Daughter who needs to seduce some manager" - maybe I don't understand Japanese culture, but that does look a bit unbelievable too - their previous problems were rather realistic, now it just looks like new problems appear that odds of happening are quite slim.
To be fair, remember that both of them have been betrayed by past relationships... it's possible that there's a hidden sense of distrust and fear to openly talk about things. I get the sense in the early chapters that both leads are trying very hard to be self-reliant after their partners have left them, so naturally they don't want to revert back to asking other people for help. Besides, would
Our main hero and heroine seems to be very childish in this relationship - I would bet my money they know next to nothing about each other, hence all that dirt keeps coming out. Because it's impossible to sit down and discuss things, to come up with plan - everything from them requires a lot of emotions and unnecessary moves. They can't decide together on catching the culprit, they can't tell each other about their life problems and they can't properly split duties in the house - that thing I can't understand and somehow it irritates me - although it is that disorganised lifestyle they have which makes us smile and is cause of their problems.