If what you meant is she didn't get a positive on a home test until December 16, certainly the guy on October 31 is ruled out. It only takes two weeks from conception for a pregnancy to appear on a home pregnancy test.
So if she had been constantly testing but December 16 was the first day she got a positive result on a pregnancy test, that date tends to suggest she got pregnant nearer the end of November, not even as early as November 18. It puts you squarely in position to be the dad. Then the question is where the November 18 date came from.
You figured the conception date from her due date. But what you would need to know is, how did she get that due date, and when in the pregnancy? Did a doctor give it to her early in her pregnancy after measuring the embryo on an ultrasound? (By early, I mean 7th or 8th week. Those are the most reliable for this purpose.) Did the due date come from an ultrasound that was later than the first trimester? (Those are less accurate; babies vary in size as they develop.) Did she get the due date from telling a doctor when her last period began, and the doctor took out a little cardboard wheel and counted from her last period? (Those are the least accurate due dates of all.) If August 11 is a little off because of using a less accurate method than the earliest possible ultrasound, then the November 18 estimate of conception date would also be a little off. Which the pregnancy test results seem to suggest.
Anyway, unless she slept with anyone else besides you between November 1 and 25, it sounds like you are the dad. Her pregnancy tests would have shown a positive by the middle of November if she had gotten pregnant in October.
To be sure and never have to wonder again, get a DNA test done when the baby is born.
The October sex is too early for this due date. The November sex is on the late side but still possible. Since she was sleeping around and might not be telling the full story, a paternity test when the baby comes is advisable.
November intercourse is more likely when she conceived . I have a child that was due August 8th and I conceived around November 15-21st of that year and my pregnancy test showed up positive mid / end December .