Welcome!
It’s not unusual for rabbits to get a bald spot in the middle of the heel. The fur around it folds back over it and it’s not always easy to see. Definitely do keep an eye on it though. The skin is healthy pink colour and not broken?

Some carpets can be a bit abrasive for rabbit feet. You could try putting some cotton bed sheets down over the areas she runs, if this is practical. That might help some. Also, keeping a rabbit a healthy weight and the nails short will help but some are just prone to sore hocks unfortunately.

In her cage, you could just have the shavings in a litterbox only and not use to line the whole cage. It’s not really necessary. Most rabbits like lying on the bare cage floor anyway. Some people have found those memory foam bathmats with microfibre covering good as a cage liner for buns with problem hocks.

Is she a larger breed? My Rhinelander has a small pink, almost bald spot each heel. I noticed when I got her several years ago and have kept an eye on it since, and it’s never developed into a sore, but I do check it very regularly just in case.

Keeping the nails trimmed correctly will help. Carpet can actually be a bit harsh on their feet. Non-pilling fleece is very nice for them.

Ah, well Rexes and mini rexes are prone to sore hocks, so it’s definitely something you’ll need to watch. They are also prone to weight gain which can compound problems with hocks , so be careful with her diet. I’ll let others comment more on the cage liner. I have a fabric chewer and can’t get away with letting her have much. I have found a sheet wrapped tightly around carpet tiles has been left alone. I have this up on top of an old set of drawers where they tend to snooze. I think because it’s flat and smooth, she tends to leave it alone.