Anytime I see the title "neuropsychologist" I turn away in disgust! But I'm mostly just needling you about physics supremacy here, so don't take me too seriously (even though in reality I do know a shit load about this topic).

the spring constant of a soccer ball doesn't tend to be a regulated quantity that's published anywhere

I have no idea what the regulations around a soccer ball are, but obviously even with a given material this is going to change with the inflation of the ball. My guess is that if you really wanted to calculate the force, you have to account for the vibration of the ball, since obviously that's going to greatly affect the conservation of momentum. Given that if you drop a soccer ball just to see how much energy is lost to the world during a collision, it's probably greater than 50%. I don't know, but that's where my gut goes.

But the much bigger problem is the shape of the head, which matters a lot, and also the strength of one's neck muscles, which also matter a lot. The latter because obviously the g force experienced by a head is going to be dependent, ultimately, on how fast the head is whipped about. Stronger neck muscles are going to limit that. It's one of the hypotheses for why women soccer players appear to have more concussions than men. The former, because the chance of getting a concussion varies pretty significantly depending where on the head the force is applied and in what direction. Man now that I'm thinking about it, you could make a really good qualifying exam question around this.