In Mishnat Sofrim, the Chofetz Chaim discusses the correct forms of the letters as they should appear in sifrei sta"m. When he comes to thee letter פ in its medial form, he says that the current practice, which he terms פ שבור (or broken פ, pictured here) is not the correct form of the letter. However, in most sifrei Torah I've seen from prior to the 1980s (read all of the ones I've seen)as well as the omnipresent KTAV tikkun, the letter פ is nonetheless written out in the manner referred to as שבור, and Chassidei Chabad write their sta"m with it to this day.
So here are my questions:
Why did the Chofetz Chaim effectively seek to passul nearly all existing sifrei sta"m of his day? Why wasn't the existing פ kosher?
Why was his פ not accepted for so many years?
What sources (outside of Otiot HaRav) are followed which disagree with his p'sak?