Great case.
This is the best example of fluid around a peroneal nerve that I have seen.
This fluid does indeed indicate that there has been a poster-lateral knee joint capsule injury.
Your images of the LCL leave me uncertain as to the exact extent of the injury as there is always a lot of anisotropy where the LCL interdigitates and passes through the biceps femurs collagen adjacent to the fibula head.
The fluid around the CPD is generated by a defect on the poster-lateral knee joint capsule. There is a series of little ligaments that can tear here causing this fluid.
The ligaments in question include:
Fabellofibular ligament
Arcuate ligament
Popliteus fibular ligament
And others!!
There is some material published that we can see these structures with ultrasound, but in practice it isn not helpful.
These patients with posterolateral corner injuries need an MRI to sort out the extent of the injury.
The fluid around the CPN and the general collagen derangement around the distal LCL both indicate that there is a tear of some of these structures which make up the posterolateral corner of the knee joint capsule.
MRI next !
Great case,
Steve