Hmm? This is indeed stressful for you - and I fully understand why and how. It's the complexities of a language that we don't fully understand, but we can kind of part read because there are some repetitive behaviours. Henry still occasionally humps Lillian even though they've both been done. By the book, he shouldn't have any desire to behave like this because he's plum-less. So it's more deep rooted. Lillian isn't a fan of such ungentlemanly behaviour and she hops off and luckily for us, that's as far as it goes. The dominance in our two is perhaps more even because neither shows much agression. Henry loves to groom Lillian and is devoted to that, and recently, in turn, she has also taken to grooming him. But she asserts dominance by coming up to him when he is minding his own business and shoving her head down in front of him. Henry on his part, has independance and will spend hours up the garden on his own, just watching the world go by. Lillian meanwhile, is lost without Henry and looks actively for him. At meal times, Henry struggles to get near the food. But they are happy and they have not fought.
So, from my seat, humping is a ritual and is a way of asserting dominance. From what you say, Grace is attempting to naturally assert dominance and probably Luther is thinking "what the hell?" and is resisting because he has not been used to this.
Now they are in familiar surroundings, they have settled and perhaps they feel they can push boundaries? Luther knows this is his home and is confident to defend that belief, whilst Grace feels she wants to be in the home, but also needs to feel superior to Luther because that's how the heirachy works.
However, you have HUGE positives in the photos - the snuggling up photos show that both are content to sit and tollerate. Look back a few weeks and it was a different story. I am a bit concerned about the fur pulling - especially on the head so I would state the obvious and say that you still need to maintain vigilance and intervene as you have been, but also as you have been, sharing in praise and of telling off. I would suggest that neither is singled out for any chastisement and that both are equally told off and both equally praised. Like children, it's important to show balance to both so that neither gets preferential treatment.
What you are doing is absolutley brilliant - and yes hugely stressful. But your love for Luther and Grace is abundant and so is your patience and support and so I feel you will see improvements in the coming days.
All rabbits are different in so many ways. No two are alike and sometimes getting a bond to work just takes a bit longer. I wish at times, we could speak directly to our buns and just reassure them that we are here as support and help, not to harm, and just make them understand that in our care, they are safe and won't be at risk.
Continuing to send vibes and hope the next few days are better. They are a lovley couple. x