Everything you are describing does point to getting pregnant from the sex August 26 (unless you had sex on the 27, 28 or 29th, and then it might be one of those).
The sex on August 4 is totally ruled out, not just because of how early it is in relation to your due date but also because of the Plan B, not to mention you had a period after. Because you said it was a normal period, it sounds like you also didn't bet wonky cycle weirdness caused by Plan B.
The event on August 21 with Guy A where you stopped before penetration, with the only contact being pre-ejaculatory fluid on your leg and stomach, is also early by at least 5 days, if not a few more days. For perfect certainty, I guess you would want it to be even earlier, but seriously if the fluid only got on your leg and stomach, and given that the ultrasound said 8w2d on October 9, it sounds like Guy A is out of the picture and you're in the clear. (From the Plan B and the changing your mind on August 21, I assume you're saying Guy A is not the dad of choice.)
In your shoes, I would go through the pregnancy assuming you got pregnant from the sex on August 26 with Guy B. If your menstrual months are completely even and your period was due September 12 or 13, you probably didn't ovulate right on August 26, it might have even been up to three days later than that. (Sperm can live that long in your body.) This all points to Guy B.
Your title is asking about a paternity test. Are you talking about the expensive kind done before the baby comes, or do you just mean ever getting one at all? If you are considering getting a prenatal test, in your shoes again, I wouldn't bother because your dates and ultrasound info sound clear enough not to spend the huge bucks. (That is, unless Guy B is interested enough to pay for it. Then, by all means.) But regarding getting a DNA test ever, yes, certainly get one. If you are not married to Guy B, you should always have one in the baby's medical record for legal reasons. (Does Guy B seem like the kind of guy who would try to back out of child-support payments?) Call the clerk at the family courts in your area of jurisdiction and find out what is required for a DNA test that is considered admissible in court (I assume if your area is like most, it's something that can be done at the hospital right after the baby comes) and be ready with the details. You can also ask your doctor for this info, but your doctor is not a lawyer. It never hurts to get good advice.
Congratulations on your upcoming baby!