You asked how accurate the scan was. Unfortunately, you do not have enough information to know. You don't mention what date she had the scan. You weren't there to hear what questions she asked the ultrasound technologist, meaning you don't know what parts of this she worked out herself from information given to her by the tech, and what parts were told to her directly by the tech. Plus, an ultrasound scan at 7 weeks 1 day has a one or two day margin for error (could be right on, could be one or two days off either direction) if used to try to determine when conception was.
This leaves you with the condom use and the DNA tests. I would assume the condoms did what they were supposed to, and would also assume the DNA tests are correct, though in the latter case you could look up the lab's name online and google whether anyone is complaining about false results.
If you cannot stop being nervous about this, offer to pay for one more DNA test, and use the DDC. They are a big lab (and I think they do testing internationally) and if they come up with the same results as the lab you used, you could rest assured that the baby is from the woman's husband.