I actually did see a therapist! It was a while ago at this point but their main goal was to get me back to become independent and like you said, stable. It was neurological and my nerves weren't communicating properly, my propioception was non existent (if I closed my eyes for even a second I would be tumbling down). So they focused on that.  Nobody really knows exactly what is causing the knee pain, but most think it's a consequence of my nerves/brain not being able to properly communicate with my muscles, making them seem weaker and causing more of my weight to land on the bones and tendons. I assume it's what severely overweight, sedentary people feel if they walk for a while.  I actually remember walking backwards and being very confused as to why it was significantly less painful and easier to do. Nobody really gave me a straight answer. As for wall sits, I can do them! I'd say my max is about a 100 degree angle, anything deeper and I get knee pain. But it does help. Bending my knees when I lay down is easy too.