Dresses can be a pain to pose. Some vendors add morphs or handles to help in the posing of the sitting position but even then it can take alot of tweaking.
Magnets is a good way to get it to look right but I'm not an expert on magnets I'm afraid.
Regarding the handles. You can dial spin them to the desired position but it's probably best at first to just grap them & pull so you have an idea of how much effect they will have. To use them, select & then drag the mouse while keeping your finger on the button.
Sorry I can't help more.