As I said, I would go through the rest of the pregnancy assuming the baby was from the September guy, not the October guy. Then test and forget it.
I think not, since your ultrasound was in your 23rd week and not your 40th week. I would get a DNA test done with both guys after the baby is born for confirmation, but would go through the rest of the pregnancy assuming the baby was from the September guy and not the October guy.