Just a brief update: we had to take a few days off from bonding due to Christmas activities, but we got the buns back together on Sun. We had a slight breakthrough, Poppy groomed Billy, BUT she expected grooming in return, and he point blank refused, so she went off in a huff with him.
I went into our living room for something, and poppy darted in while the door was open, so we had to keep it open. In the end, both rabbits enjoyed the free access to the hall and living room though, so we kept it open for a few hours. They spent some time together in the hall, they spent some time apart. At different points they had a mad dash to let out some energy (they love having the hall door open so they can sprint back and forth), and we saw no fights. Still a touch of nipping though.