Bunnies can hold grudges, so since Mufasa and Guiness fought, he is likely hanging on to those bad feelings.
You can try a long break (3 months or so) with no contact between them (so sight, minimize smells), to see if he can forget. You also didn’t describe the bonding space you used, but very large, very neutral spaces can be helpful. In small spaces bunnies that are afraid feel they have no choice but to fight.
If you are open to having 2 separate pairs, I would probably give up on bonding Guiness with the other two, and see if you can take him on some dates at a local rescue. That would give you a sense as to whether he would prefer to be a solo bun, or whether you just haven’t found the right match.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.