Thought it would have been 10 x better if... (Spoiler)
...they just left it as the Mum being crazy and that she had imagined/created all the dodgy things the downstairs neighbours did. It would have been horribly tragic, don't get me wrong... if she had done the unthinkable to her baby and killed herself - but it would have been much more unpredictable and impactful in my opinion, especially the bet where she sent the email saying she's sorry and the husbands reaction, really powerful. Having the final 'twist' of the odd couple setting it all up, made it feel like a run of the mill OK thriller which was very contrived.
Still didn't mind it but that's just my opinion.