A lot of this is in those posts and docs.
Forms - not sure you can guarantee this given the rules around dupes, record creation date. How would you know which one to attach it to with unique API code?
We were hoping to add an additional identifier, rather than just email, to match a form fill to a person.
Our duplicates are coming from a different brand, so in a perfect world the situation would be:
if form fill is for brand A, assign to firstname.lastname@example.org with Brand = brand A.
if form fill is for brand B, assign to email@example.com with Brand = brand B.
I'm not sure what you mean by unique API code. How does that work?