I know it can’t be this because I’m sure your vet would’ve ruled it out straight away as I’m assuming it would be obvious on the X-ray so this is probably of no help it’s just when I read your description of Harley’s hock being almost on the ground when she walks plus the lump it sounded like you were describing a ruptured/torn Achilles’ tendon.
This is an interesting observation. So I want to share the following in case it helps:
When our house cleaner slipped, she hurt her leg. She had an X-ray at the hospital and there was no broken bone and they sent her home with pain meds. It didn’t get better.
We were away for a few weeks when this happened. Unfortunately she has no family in our city to help. When we got back, I took her to a private GP (the GP I went to personally) who said she was complaining and she should just wear an ankle or knee guard (I forget which). When I asked her if he’d actually examined her, she said no. I was shocked. I stopped going to him, I was so furious!
She said she had a lump on the back of her leg (calf area). I asked her if I could feel up the back of her leg. As I ran my hand from knee to ankle, there was a sudden dip above her heel where you’d ordinarily feel a strong tendon. A ruptured Achilles.
I then took her to a private physio who thoroughly examined her and diagnosed a ruptured Achilles but hanging on by a thread, hence she could still walk.
With this diagnosis, she got scheduled for an operation and thankfully had one of the best surgeons for this type of operation, who happened to be on rotation at the hospital at the time, and he fixed it! He cut the Achilles in half lengthwise and folded back one half and attached it to her heel. Because it had bunched up into a ball, and because it hadn’t been treated immediately, he wasn’t able to stretch it, hence the lengthwise cut (I think it’s called a Z cut). She wasn’t able to walk on it for about 3 months after that, and had physio. It’s now about 7 years later and she’s walking long distances and has no problems.