World Wide Photos was around after 1954. I have a 1960s photo with a World Wide Photo stamp on back. If a "1920-54" time period stamp is on a 1957 photo, that would say that "1920-54" dates were incorrect.
I do think there are details about the stamps, including when exactly they were used, that is currently unknown. Future adjustments may happen-- though I don't expect radical ones. I think the current published dates are generally correct.
The 2 year rule is arbitrary. It's not a number I would chose-- though I was never asked
I don't think it's a horrible number, just arbitrary. It could as easily be something different.