1. This is an on-going debate to a certain degree (I mainly have encountered it via Phillipe Ariès's
Centuries of Childhood). But I have never bought the argument that childhood is a modern invention. If you actually read primary sources from the period it is abundantly clear that parents sought to protect their children, that they treated children as different from adults, and that children were expected to behave in a manner different from adults. To me, that's a childhood, albeit one that looks very different from modern norms due to the existence of widespread child labor, etc.
2. It's too strong of a claim to say that medieval or early modern warriors suffered from CTE (mostly because retroactive diagnosis has a terrible track record) but I think it's very reasonable to say that warriors of this period suffered from severe psychological and physical distress that derived directly from their training. Likewise, you're leaving out the fact that nobles hunted and rode on horseback (with no helmet!) on a daily basis and that these are exceptionally dangerous occupations by modern standards.
3. Smallpox might last a matter of weeks as an active illness. But it is permanently disfiguring. Elizabeth I is just the most well-documented of what we can imagine were literally millions of people who suffered trauma from smallpox scars. Similarly, many illnesses of the period (like malaria, polio, scrofula, etc) were chronic. As for gangrenous limbs, Henry VIII suffered from a wound (from recreational jousting no less) that ultimately killed him via a leg ulcer, but it took years of agonizing pain to do so. Maybe not gangrene, but the point remains that a world of sharp objects + no antibiotics is no fun for anyone.
4. It's very difficult for me to imagine that the pain of losing a child ever really goes away, even if you live in a world where it's commonplace. Likewise, the evidence about children not being considered "fully human" until age 1 relies on naming practices. But it ignores things like murder prosecutions for infanticide which were widespread in the period - if babies weren't considered fully human, why did courts care if they were killed? And finally, most childhood mortality occurred after age 1, regardless.
Edit: I mostly agree with your critique of the OP, by the way. But I think the fact that we agree on humans from the past being more or less like us conflicts with your points about parents in the past being relatively un-concerned about the loss of a child, etc.