I like reading into fiction to find symbolism. I don't really understand or fully grasp the connection between the symbolism you found & the term "traditional primal methods." This excerpt didn't connect the dots for me well enough:

Zombies are a narrative trend in which the stories tend to have one common element: the disruption and failure of authority due to the primal reactions of survivors. It’s not just that an apocalyptic event took out society, but that the survivors who remain refuse to continue moral policies and regress to a more primitive state.

First, the disruption and failure of authority in society is the result of zombies first and foremost, not survivor behavior.

You suggest that survivors do not continue to uphold social moral policies so they're "regressing" to "a more primitive state" once society has collapsed. In truth though, they're no less primitive (or advanced) than before society collapsed. Society is primal & instinctive to humans. We seek cooperation & mutually beneficial interactions with one another. When the whole global mainframe of that (one of the most) basic tenet(s) of our species collapses, humans are more compromised, sure, and humans may do things to survive that they couldn't imagine beforehand, but humans don't lose their moral compasses & they don't lose their cognitive abilities to make cost benefit analyses... and they definitely don't lose the desire to ally with others.

This story and its symbolism that you talk about in this post - it's about adaptation, not regression. Survival of the fittest has always and will always be about a species' ability to adapt to new or unexpected external stimuli. Human society has always been about cooperation in order to weather new or unexpected external stimuli. When society collapses, humans must simply adapt & find new ways to cooperate & survive.

Nick, as a social reject (a homeless addict), adapts to the post-apoc pretty easily since he never relied upon society to protect him to begin with.

Daniel was victimized by a corrupt society (btw corrupt societies are a great reason why we should refrain from associating society or civilization or government to higher moral policies); attempted to adapt to the post-apoc by relying on the skills he gained from those past experiences... and it ate him up bc those skills were immoral & brutal. El Salvadore's civil war corrupted him & his morals, and the post-apoc brought that back to him & he couldn't live with it. So I don't think the post-apoc or any regression to base primitive human instincts destroyed him - I think it was his own psychological torment that occurred during the Salvadoran civil war (people - just people - before there were ever any zombies around) that ruined him & his ability to adapt to the post-apoc.

Chris - You wrote "Chris is the symbol that the post-outbreak world will have significant consequences for the future of humanity" which is kind of so vague that it could work for any character in this series.

I honestly don't know what Chris represents exactly. I think he thinks he's turning into an evil person. And I think he's doing that to himself as a way to hurt his father. Even though I think he's rationalizing it that he's just preparing & gearing up for this post-apocalyptic New World. Maybe Chris is the symbol that shattering one's moral compass, even in the post-apoc, is not conducive to adapting or surviving.