Is there a disagreement on the popular date of 12 Rabi' Al-Awwal among scholars or among pro-Mawlid Muslims? Are the Fatimids the origin of the date of 12 Rabi' Al-Awwal?
There is no agreement on Prophet's (peace be upon him) birthday. The Fatimids have started Prophet's birthday celebration, but they didn't invent the date. The date 12th Rabi' Al-Awwal is not totally rejected by Sunni Scholars, meaning it is a possible date.