Humping when an animal has been spayed/neutered is about asserting dominance. So it sounds like Lilli is trying to establish that she is in charge now, and Fender disagrees. The shouldering and face to face behavior are control behaviors as well. It's a rough form of 'herding' him. Holding him in one place, and going for him if he moves is a pretty common behavior in BCs to control another animal.
First, you can't let it continue or will be become obsessive behavior with anything that moves. You can't obilterate the drive, but you can teach her that it's up to you whether, or when, she does it. Teach her a 'that'll do' or 'leave it' command. You will have to be consistent, and patient. Any time she gets away with it only reinforces it. Then you will have to give her something else to do. You can't just say 'no' and leave her hanging - she needs to know what you DO want her to do. So, have her do a little trick, such as sit, go to place, down -whatever you like - then praise her effusively so that in her mind this new behavior is much more rewarding.
It takes time, but it will be rewarding and bring you closer. They want to do what you want them to do, they just need clear direction.
We have 2 BCs and 6 cats. We have spent many an hour working on this, but our BCs and cats live in peace for the most part now.
Good luck!