Rule number of melodrama, logic leaps are a necessity to enjoy the show.
Plot shields are a given.
If an action / scene is repeated, the shield is there.
All agents are identified and known, by order of importance.
The protagonist has to be likeable however deproable her actions may be.
Everything has meaning, but never overtly so. For the show is to the masses.
#1, I vote Go Ah Jung to the role of mastermind. her considerable screen time do no justice to the filler character she had been portraying. Throughout the show, she had been seen surveying the family with all her secret cameras, devices. That plot shield grants her omniscient, elevating her importance.
#2, the average audience is looking to fill his time, not to brain teasers. Odds are there is no hidden agenda. The purpose of that initial meeting has been addressed by Tae in Joon as the spark that kicked off his romance for Go Hae Ra. (He like her because she's warm. Not cold and calculating, but rather because she's hot with emotions.) (Initial meeting to just to call attention to her personality)
#4, a decent script writer should always aim for a reasonable ending. Good or bad, the audience should be able to come to terms with. Ideally, a happy ending with all the bad people paying for their deeds. For the world is a cruel place, and I do not need that from a drama.
#another thought. Korean drama has a tendency to typecast TV personalities. A celebrity famous for a particularly character would play a yet another similar character. Apparently Lee min Jung has always played the pitiful wife but always have a decently happy ending. So Yi Hyun is famous playing the role of a good daughter in law, so I would suppose the drama would not deny her a chance at redemption. In fact, she has not done any irredeemable yet. And the daughter always needs a new kidney
