For the first part, I already said that if the relations were gone then Yudhishtir didn't have the right to stake her(which Arjun agreed to as well) making Yudhishtir as much of a criminal as DDKS. I would personally go with this stance
If Yudhishtir had the right(i.e. Yudhishtir was at no crime) then she was both a sister in law and a Dasi, she had dual relation with Duryodhan now, it was upto Duryodhan on which relation he carries out and when, since by the second relation, he was the master and therefore had greater say. While in the hall he executed the master slave relation and not the BIL SIL relation. It was his choice. Hence he doesn't become a criminal either. That is the reason no one in the hall actually could stop either of them from doing all that they did. You can not expect one to always treat a Dasi with the same respect, you would do to a Sister in law. Most in the royal court took this stand I guess and hence no one stopped Dury+gang till Dury himself declared
There could be no denying the fact that Draupadi played her cards pretty well there and responded with rationale and presence of mind.
But this does not change the fact that all five (Yudhishtir+DDKS) were equally at fault. Either they were all criminals or they all were not criminals