I agree with all of what you said! I felt like the show was hinting at Do Guk having been in love with her for a while. 

Additionally, I think Yo Ju's dad got off way too easily considering that he was ok with her being abused at his house for years. 

KeenRa:

I was disappointed that they never revealed when Seo Do-guk (ml) fell in love with Han Yi-joo (fl) because before they went back to the past he was already in love with her as far as it seemed unless i got it wrong…

But Seo Do-guk said to Han Yo-joo’s ex fiancé he has loved her for a long time and that she can use him… 

Last half of episode 11 and episode 12 i feel were just drag they could have used last half of episode 11 or first half of episode 12 to get that story in.. 

another thing is no one even once gave it a thought that when fl’s step-sister said then im the fake.. Han Yo-joo was a bit puzzled but I’m surprised it didn’t even hit her to think about it.. or look into it they had 1 1/2 episode yet 2 things were never unfolded😒

They clarified the first point-at least, that's how I interpreted it: Seo Do-guk developed feelings for her the moment he found out she was the artist behind the painting, but she was already married by then. He indirectly explains this to her on the couch in episode 11, when he mentions that he agreed to the initial arrangement because he realized he had been in love with her all along.