If an ultrasound at 14 weeks showed your due date as Oct 2 and an ultrasound at 20 weeks showed the same but a 28-week scan gave a Sept 23 due date could you have conceived before Dec 12?

Absolutely not. The later in pregnancy the less accurate the scan. The first two giving a due date of October 2nd will be correct and give you a conception date of around January 9th.