I'd trust the early ultrasound and its follow-up; it sounds like you got pregnant just before having the sex with the wrong guy. (Good thing you didn't have sex with the wrong guy just before, instead of just after, the date it sounds like you got pregnant.) Get a DNA test with Mr. Wrong once the baby is born so you won't worry about this your whole life, but it sounds like with your 7w2d ultrasound saying you were due October 11, you ovulated around the 18th of January.
Regarding freaking out such that you can't eat or sleep, even if the baby weren't from your partner, that is awfully self-indulgent behavior when you're carrying a baby. I take it you never had to deal with an ambiguous situation that is serious and in which you felt culpable, before? Dumping a load of stress on, and not eating, and moaning and flagellating yourself doesn't make it better. Who the baby's father is, is already set in stone. There are only two possible realities, one that the baby came from Mr. Wrong and the other that the baby came from Mr. Right, you moaning and being anxious will not change the baby's DNA. A mommy's duty is to protect and nurture her child, there is no excuse for not eating and sleeping. You may have worries for yourself and your relationship with your boyfriend, but your largest duty from now until the day you die is to your child. It overrides all concerns, even this concern. And given that it looks like Mr. Wrong was three days too late to be the dad, working yourself into a frazzle is a massive waste of your emotional energy, too.
I get that you wish the doubt would go away and wish the event would have gone away. But the event happened, and you've done everything you can in terms of medical evidence short of a prenatal DNA test to let the doubt go away. So, either get a prenatal test from Ravgen or the DDC (with both guys if you can) or just pull up your socks and take care of yourself. Your life won't end if your worst fears are realized, there are women in the Infertility and Miscarriages communities who would trade places with you right now in a heartbeat even with not knowing. And it looks like your fears aren't going to be realized. So get some sleep and get something to eat. If you just can't cope with the ambiguity even realizing all of this, see a therapist. It can do a world of good just to talk things over with a trained person.
Also, my last period was December 24-26. I also had some brownish discharge/ spotting in January on the 23-25.