Nov 22 would have been day 12 of your cycle.
Dec 18 would have been day 10 of your cycle.
You'd normally ovulate between days 11 and 16, sperm can survive about 5 days.
If you had a "normal" period on Dec 8 then it is very unlikely you were pregnant then and the 2nd guy would be the father.
Yes, the ultrasound will be able to determine the 4 week difference.