A pretty tangled relationship, certainly.
First, to answer Jan, I have to say I liked Jacob. Perhaps he was a man of impulse, but that doesn't seem so bad to me. Marlena knocked him over, and all thoughts of what was right or wrong were quickly put aside. He was head over heels in love with her - who thinks straight in that situation?
Besides being mentally ill, so that when he got mad, he went crazy, August could be a charming man, but he was also an egotistical man. I think he had Jacob over for the first dinner just to show off how good he had it, including being married to Marlena. I'm sure he thought he could keep control of the situation.
I guess Marlena was just very unhappy with her situation, and saw Jacob as a way out of it. I don't understand why she invited Jacob to the celebration either. Maybe she wanted to celebrate with someone she liked for a change. She was the one person who could predict that things were going to get ugly sooner or later, but I think she was so tired of life with August, she wanted something to happen.
Steve