Hehe, “Loulou seemed to enjoy this a lot and had a very low and submissive stance”, when the bunny getting groomed has their head down like that, it is them demanding that the underling groom them. So she wasnt being submissive here, she was being dominant
When you say he wouldnt let her groom in return, what do you mean?
It sounds like they are both very headstrong about being the dom and neither wants to be the sub to eachother. Since they are both spraying and peeing in the same spots, they are also fighting for territory. I would keep on with the same amount of sessions you are having, for the same time limit. I would increase the time they spend together only when it starts to become obvious that one is willing to back down and submit. Try not to intervene aside from to stop scuffles and stop any chasing. Some bonds can take a few weeks, while other bonds take a year or longer to fully cement. If after a few months they do nothing but fight about who is the alpha and neither show any signs of submitting, it might be best not to bond them to avoid further fighting and avoid possible injury