Black bar means shutter is too fast for the flash and you need to enable HIgh Speed Sync.
When you are at 2.8 your depth of field is narrow. At 10 feet away, your range is 9.5 to 10.6 for what will be in focus. That is only 1ft. Shooting at 5.6 puts it at 2ft.
I think you had a compound issue. Increase your f-stop and ISO. Drop your shutter speed, because the flash will stop do a good job of freezing the motion. Increase your flash power fill the frame.
The paper is a professional business and knows they need to credit photographers. Some have just become lazy, and no one bothers them anymore, so they get away with it. They get to use your stuff to make money.
Send them a bill. If it isn't paid, then small claims court. No newspaper wants to be seen as the bully and ripping people off.
You can share on social media the post and with a statement how they used your photos without permission or credit.
Just remember, you own the copyright, and you give usage license under terms. The terms were not met, so you have legal rights to compensation.
If you're a photographer in the United States, the copyright is based on who pushed the button on the camera. So legally, she owns the copyright of the photos she took of you. This was a big ordeal when a chimpanzee took a photo and there was a court case about who owned the copyright.
However, there was a certain level of expectations with a photo walk. That being you two consented to having the pictures taken, didn't matter if it was in public or private. In addition, with the social norm being that photos are usually posted on social media, then it is reasonable this usage was implied when you offered a photo walk date.
That being said, I wouldn't use then commercially to promote your photography, but personally, it wouldn't be an issue.