Humping has reason and in this context it is dominance related and its severity/frequency is likely heightened since Elvira not spayed. Fixed rabbits will often hump during bonding because that is a hierarchical behavior and a part of their communication. Also, please note the occurrence of a false pregnancy doesn’t necessarily need to be contingent with Biscuit. False pregnancies are something intact female rabbits go through, regardless if there is another rabbit around or not: https://wabbitwiki.com/wiki/False_pregnancy , though yes, Biscuits presence and behavior likely doesn’t help.
It is common for rabbits to appear bonded across pen bars, yet not get along once removed. Pen bars are a solid, immovable territory marker that clearly delineate space and boundaries. Bonded rabbits willingly share space with each other. Across pen bars, they are not sharing space at all. The dynamics of the settings are extremely different, even though it seems like the presence of a thin pen wall is small– it’s doing a lot.
To be honest, not every pair of rabbits are compatible and one being intact works against you. Hormones are unpredictable and there’s no way to know when or how they may impact the bond dynamic. The bond between two fixed rabbits is much more likely to succeed and be stable than that of an intact/fixed pairing. The territorial/mating urges generated in either rabbit that are amplified by the presence of hormones are not easy and sometimes not possible to overcome.
That being said, you can put your best foot forward to try techniques to make the bond work, but again, understanding that trying to create a stable bond with one rabbit being intact is difficult and potentially impossible, depending on the rabbits: https://binkybunny.com/infocategory/bonding/
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.