How long before you put them together after your boy was neutered? I only ask because boys can retain sperm for 4 weeks, sometimes up to 6 weeks post-neuter.
As for the mounting, rabbits of both sexes do it. It’s a dominance thing. One bunny will try humping the other to say they are top bun (or to test the waters, so to speak). When the humpee doesn’t agree with the idea, then they will turn the tables. It can turn into a battle of sorts until someone finally decides to step down.
That being said, your girl is probably hormonal too. Hormones can do some pretty strange things, making bunnies aggressive at times and it is difficult for them to form a bond. Your girl might be switching back and forth between concerns about dominance and concerns about having babies. That’s why it’s a good idea to have everyone spayed/neutered before there are any attempts at bonding.
I know you’ve probably heard this already, but it really is in your girl’s best interest to have her spayed. Not only will her hormones create obstacles to bonding, but she is at risk of cancer. Depending on genetics, females have a 50-80% chance of developing uterine cancer by age 4.