I may be the only one who feels like this but oh well.
You wanted to try opening the marriage with no promises of loving it. Perfectly fine. Do not let anyone tell you differently. You put forth the effort and tried to be okay and go with it. You guys attended counselling and opted to close the marriage in the hopes of repairing it.
Here is my problem. Your husband may not be physically cheating, but emotional affairs do exist and can cause way more havoc than if he had just had meaningless sex with her. If your husband is serious and committed to working on the marriage, it requires his attention and effort. If he has still caught up in his feelings and if they are anything reminiscent of NRE, there is no way he is thinking clearly. Have you told your husband how you really feel? If she respects you and your marriage, she should also be willing to make concessions. I am never one to say how people feel in weeks or months time, but how deep are those feelings? I am not saying you have to give him an ultimatum but be honest and let him know that you think/feel that he is not giving it his all due to lingering feelings for her. Only if that is the case.
Why is he being bitter and hostile towards you? Did you do something to him? He needs to get his attitude in check and deal with his issues. He had to agree to close the marriage. Did you tie him down and force him to tell her that their relationship must come to an end? Obviously he has some internal stuff going on, but that should not stop him from being nice or cordial towards you. He reminds me of my four year old who pitches a fit when she cannot have her way.