In response to the original question, of a Main Character returning to a hometown and being impacted by their memories of a place…
Assuming the Main Character’s story is about a source of conflict generated from what happened (the Past), the Influence Character perspective would be anyone who triggers recollections through conflict.
The Main Character is not a real person, she is a perspective, so it’s not her Memories that are being triggered or brought to light, but rather a process of Memory-ing (recollecting, reminiscing) that is the source of challenge to the personal perspective.
Flashbacks may be part of this Memory process, but Backstory is not. Backstory is where you find the genesis of the justification in the current story:
Backstory -> Initial Story Driver -> Forestory
Forestory is what most people understand to be THE story. Backstory explains WHY, or HOW, the individual Problems in each Throughline came to be.
As awesome as those PRCO inequities are in @JohnDusenberry 's post above, I’m not sure the sequence as presented would work, in that seeing him as a child feels more like Backstory (they feel really out of place with the others)
If there was a greater context of Memory - and all four were geared towards challenging through Memory (like the sequence in Inception where Cobb recounts the secret his wife Mal had hidden away), then it could work. It’s difficult without greater context.
With all that said, 9 times out of 10 when the writer is trying to write an Influence Character that is not present, or is a rock, or is someone who died a long time ago that they’re thinking about – it’s usually indicative of a writer avoiding creating an effective Main Character and Influence Character dynamic.