2. I don't think a comparison between cultures where horse-riding is fundamental to one where it is primarily a leisure activity makes much sense when it comes to estimations of danger. Yes, hunts could result in injuries; I don't disagree there. However, if attrition was severe enough to impede the function of the martial culture, it wouldn't be actively propagated.
3. Yes, disfigurements and scarring would be common in the aftermath of epidemics, but in those situations, it's so common that to form cultural persecuting complexes around it is difficult to justify. Lepers, on the other hand...
4. That's really my entire point: the reason it's so hard for you to imagine is because, if you live in the United States, the entire existence of death has become divorced from culture and daily existence. It's an aberration and a taboo. In a culture where death during childhood is common, there are cultural forms that account for it. That doesn't mean that it is or isn't a loss, but that it accomodates and integrates that event within a broader chain of being. As far as judicial injunctions against infanticide go, early mortality is incidental to life, not fundamental. Actively destroying a potential person, particularly within a truly local culture, when it isn't already part of the folkways, is of course going to result in reprisal. However the very existence of cultures that practiced exposure shows that it isn't fundamental to the human condition to viscerally value the lives of infants unconditionally. As far as PTSD goes, it's largely about being unable to cope with a traumatic event. In a functional society, it seems unlikely to me that PTSD strictly as a pathology surrounding child loss would be a thing--and the sociology surrounding it as I've read it supports that assertion.
I really think our disagreement resides in our perceptions of how durable/fragile humans can be under duress. I think humans are especially resilient, particularly if given adequate cultural coping mechanisms within a culture.