I believe you dropped your point on the evolutionary change of the mandibular foramen again. Thanks for giving me that point.
Now for fat. Gee, we have a mammal that is living in a marine environment. What is one very common physiological characteristic of mammals in these environments? Fat. Whale blubber. The creatures have it everywhere in their bodies. It acts as an insulator, energy store, provides boyancy, and even helps dolphins to whip their tales more efficiently and even aids in hearing.
As I said earlier whale that happen to have fat in the right places have a selective advantage. The nerves are already in the location. If they can more effectively pick up vibrations, it is to the advantage of that individual. Even a small improvement is advantageous.
This is not fantasy. It makes sense especially when you note that both the mandibular foramen and the ear bones show dramatic change during the same time period.