Is 'would have' always the third conditional?

It's your "modal of likely truth".

In the second sentence of your paragraph 'a' that is obvious, Stone Age children needed to chew their food more than modern children.

In the first sentence the exclusion of all Stone Age children from the group of those eating processed food appears to contradict it but it is of the same form as

"Any Stone Age ten-year-old would have been living on foraged plant matter, fresh hunted meat and, probably, some form of jerky".

This is the likely truth.

Replacing "any" with "no" and a list of Stone Age food with a list of junk food does not change the grammatical form or the likelihood of the statement's being true. It reverses the argument to say what Stone Age children were not eating but saying that Stone Age children did not live on junk food is still the likely truth.