Nice case Peter,
It is also nice that you can see the normal and intact plantaris on your short axis images.
It was not that many years ago that a history of feeling a pop while playing squash would have led to an immediate sonographic diagnosis of a plantaris tear!
In your case there is plenty of bull nose retraction of the distal medial gastrocnemius muscle belly and this is typical of an FGA failure.
I think of it like a spectrum, where the point of failure gets meow serious as you move down the list
Least to greatest significance of injury order is:
1: Medial gastroc separates from its aponeurosis and the aponeurosis remains intact.
2: Medial gastroc separates from its aponeurosis and the aponeurosis has been torn.
3: FGA failure
4: Achilles tendon rupture.
It may be that more than one failure occurs, but the management will be predicated on the most serious point of failure.
So in your case I think they will be treated as an FGA failure and nobody will care too much if some of the medial gastroc fibres were tugged from the aponeurosis in the process.
Steve.