Please don't ever force yourself to try painful physio! Pain is your body's main "early warning system" and really deserves to be heeded.
I'm sorry about your knee and the delays!!!
The thing about your hip-knee system is this: when we live with bad arthritis in a weight bearing joint like a hip, over extended time it throws off all the surrounding soft tissues as they (mal)adapt to compensate.
As a result, muscles, tendons, and ligaments change - some tighten and become contracted while their counterparts stretch and become lax. Over time, because those structures attatch up to pelvis-spine and down to the knee, this affects those areas too.
The pelvis can get skewed in up to three planes, creating hip, sacrum, and back issues. And yes the pressures on the knee could be destabilizing - even to the point of messing with the medial or lateral meniscus.
Here at BoneSmart we often see this following joint replacement, when in surgery everything is suddenly reset. But it's not uncommon to be symptomatic preop too.