I can answer your question very easily, and it has absolutely nothing to do with phrenology outside of the fact that what you describe as an expanded "galea" which I cannot find any clinical definition for, is the result of high levels of growth hormone.
When you extrapolate that against the fact that those people might possibly have persistently high levels of growth hormone throughout their lives, and the fact that growth hormone in excess leads to insulin resistance and diabetes, you start to see a pattern. Mitigate the excess of growth hormone, mitigate the hair loss, high GH itself can cause hair loss but also by means of inducing insulin resistance.
It's not the shape of the head, it's the hormonal profile which developmentally may have been the cause of it, at least have a solid understanding of why craniofacial development occurs and its relationship to hormones and what effects those hormones actually have.
I experienced significant, in fact a scary amount of embarrassing hair loss that I recovered from in my 20s and my forehead/crown is not wide and extremely horizontal and my hair recovered amazingly and I had returned to a dense mop on my head for most of my 30s.
Phrenology/physiognomy is the religion of people either too lazy or not intelligent enough to discern the root causes of physical traits and instead try to correlate things based on their own arbitrary mythology instead of looking at what biologically causes what.
Frankly you sound like that nutjob from 8chan pol from three years ago that tried saying that people who didn't have detached earlobes was a sign of pedophilia, what you're describing if you knew what you were talking about is about as insane.