It sounded like Ethical man and Katz were friends and EM hired (or asked) Katz to investigate the blackmail. His secret being revealed would 'bomb' his 'ethicaI whatzit' app which was aimed at 'church-going' investors. EM said that he'd put 'everything' into the app and if it failed, he'd be ruined.
I think that Katz killed Heidi after he realized that she knew Joanne. I suppose he feared that Heidi would tell Joanne about him and that somehow Jo would connect the dots, or would figure out who 'ethical man' was and EM would implicate him.
I thought the 'check the DNA' thing was just to make Adam take the fake pregnancy thing more seriously, to make him feel he had more reason to not trust Corinne.
I think the 'Stranger' ending up being Adam's half-sister was overegging it a bit. But I did like the part about Stephen Rea stuffing his wife in the wall. That was unexpected! The only thing was that I swear that he'd previously referred to his child (Chris) as a 'he'.
And I think Tripp was an example of someone who felt that he was entitled to do anything he felt he had to in order to hold on to his 'position'. In short, a sociopath. And I loved that Jo helped Adam get away with killing him. Not at all legal, but sometimes it's nice when the injured party gets their revenge.
I did enjoy the series. Sometimes it veered into "Really?" territory but by and large it was an enthralling watch.