This resulted in bilateral damage to her amygdala
Honestly this bit right here is probably the key as to why this woman doesn't experience PTSD and is unaffected by the trauma. The amygdala is a crucial brain region for determining emotional responses to stimuli (especially fear) and contextualising memories and emotions. People with dampened amygdala activation typically have less fear responses, to the point where some people with amygdala lesions can't experience most types of fear at all or perceive situations as life-threatening.
I'm pretty sure SM has Urbach-Wiethe syndrome by how you described it, meaning that she is most likely immune to fear conditioning due to calcification of the amygdala. This condition is extremely rare and has only occured in about 400 people in the entire world. Despite the other problems caused by the disorder, like cortical lesions resulting in epilepsy, having a damaged amygdala probably protected her from developing PTSD.
There is even a clinical trial at Yale university right now investigating how MDMA can decrease activation of the amygdala and how this can be therapeutic for people with PTSD. If anyone is interested this is their hypothesis and study protocol:
https://classic.clinicaltrials.gov/ct2/show/NCT03752918
Unfortunately, we are living in an era where neuroscience is very slow and lacking resolution. Outside of a handful of research groups, no one has the technology to conduct functional imaging of brain regions. An MRI or CT can only show extreme structural damage like lesions. An fMRI can tell you how oxygenated and activated certain parts of the brain are and give clues about functional changes, but it is only used in research, not clinical practice.
There are really no treatments or drugs out there anyway that can target this specific neuronal response of amygdala hyperactivity (except maybe MDMA and sometimes changes don't stick) despite it being a huge component of why PTSD develops. There are also some people whose connections do not end up wiring as strongly and have more capacity for neuroplasticity and circuit remodeling. So everyone can be different like you said, and yet there is a very one size fits all approach to mental suffering that doesn't really address the actual mechanism happening to cause xyz response.
I think it's hard for people to realize this, the changes that determine someone's reaction to an event in life are happening at a complex, microscopic cellular level that is difficult to measure. It's also hard to fathom how different we all are in terms of the millions of cellular interactions and connections taking place everyday.
A lot of suffering is perceived as such or compared because of averages, or what's construed as being a "wild type/normal" response, despite the vast heterogeneity in neuronal connectivity in people. There's societal conditioning about what is traumatic and what isn't, when in reality it's like you say, anything can be traumatic (or not) if the brain processes it as such. What bothers one person may be like water off the duck's back to someone else and vice versa.
I have seen a lot of dumb ass psychiatrists lately arguing on medical forums about the validity of complex PTSD, and trying to claim that it isn't a real thing because repeated non-life threatening abuses and neglect throughout life aren't actually traumatic enough to warrant PTSD like phenotypes, only what is known as the "big T traumas" (God I hate therapy speak like this). This just shows how out of touch a lot of "experts" are with the organ they should be a master of, and fundamentally don't understand how the brain works.
The sooner it's accepted that it's not about what happened but how you perceive and process the event and the world around you, the better. Until then I think there is going to be a lot of infighting about what constitutes as traumatic or upsetting enough to warrant xyz response, because there is still so much gatekeeping in the mental health community or trying to project one's own experience onto someone else.
Fundamentally only we truly know ourselves and what we've experienced, what we want, don't want, etc. Others can make guesses and attempts to understand but sometimes it's like shooting fish in a barrel.