Bonding Behaviour - Can I do anything to help them along?

We have had Mulan for a week now and she met my bereaved bunny Benjamin last Sunday for the first time. It was a good first meeting - a couple of very minor nips and then sitting close together. I've given them a chance to get together most days since and it was going quite well...at first the nips continued but then they were progressively sitting closer to one another. Benjamin has been presenting himself for grooming but Mulan will not oblige and has not groomed him at all - he has groomed her a couple of times to see if he can get her to reciprocate. Yesterday though his behaviour changed and he began mounting (something he has never done previously - and this will be his third bond now) and hair pulling. Mulan is completely submissive and allows him to hump and pull her hair until I make it stop - he could continue for hours and it is all he is interested in but she does not retaliate whatsoever. A couple of time she has dashed away when his grip has loosened but generally she just lets it happen. She will occasionally bow her head for grooming on the couple of rare occasions in the last couple of days when the humping has stopped (for him to have a rest - he is 9 and a little geriatric!) and he usually will give her a little groom and then presents himself for grooming - which is always refused from her! He is so desperate to be groomed that even when she is cleaning or washing her face he will shove his head right under her belly to see if he can get her to give him a groom but she will not.
Can anyone interpret this behaviour. I get it that Benjamins humping is his way of exerting his authority and the fact that Mulan stands it without arguing seems that she is submissive but I don't understand then why she will not groom him. I think he would probably stop the humping if she would oblige with the grooming.
Any ideas guys?

Bumpin it up for you

I'm bonding a trio at the moment and I'm finding a little smear of Ella's kitchen strawberry and apple baby food on the head/ears does the trick! I have tried various types of baby food/treats over the years and am finding this one the best - both for bunny enthusiasm for licking it off and because it isn't too sticky so it doesn't make the fur too icky like some of them can.

One good thing I have noticed - no fighting. Just keep going as you are. Rabbits have strange ways at times but in the end I think you will be successful. My rabbits try to get groomed but quite often it doesn't work as the other one is busy grooming him/herself and will only oblige if in the right mood.

Thanks everyone. I have decided that when I am off work next week I am going to set up my spare bedroom as a temp home and they can move in there for as long as it takes to get the bond cemented. I think they will do it and it's nice neutral territory. Once I'm sure they are bonded in there though is it then safe to move them back out to the shed? That's a worry for me because it's always been Benjamin's domain and he is evry protective of it!

You will need to thoroughly neutralise it all, every single inch, with a distilled vinegar solution to eradicate his scent.

You may also need to minise how much space they can access and gradually increase it, depending on what you're seeing.

Good luck

I need to say a very huuuuge thank you to Santa re the ellas kitchen strawberry and apple baby food because it has worked!!!!! Benjamin wasnt that bothered for it but Mulan has gone crazy for it!!! A drop or two on his head and shes more than willing to lick it off very enthusuastically! When she first did it Benjamins face was a picture!!!! Disbelief but also a little puzzled as to why she was being a little rough. great stuff!!!