I'm pretty sure that the mastermind behind everything was Yoona, tho I don't know about her motive: is just simply make her man jealous or... ?
currently, the hints we have are:
Yoona route
1. Fake smile of Se-Ryeon, the golden retriever
2. Cunning smile of Yoona on that dinner
3. 'deliberately' lying to Se-Ryeon about Yoojin
Yoojin route
1. 'deliberate' notion of sending the retriever to Yoojin
2. How she left that kigurumi in Yoojin place
okay... this is going to be a rather tense arc