You said it yourself, the story about dogs was just a rumor on Chinese Weibo, so how does it make sense to undermine SK media over that story?
As for the ex-girlfriend's story, the fact that the girl, a popular singer, has not been seen since lends pretty strong credence to the story that she was executed.
As for the Sino Weibo rumor about the dogs. The South Korean press and especially The Chosun Ilbo was the first to pick it up. Even when the source is just a rumor on social media. And that's how it got spread across the world.
As for Hyon Song-wol the ex-girlfriend not appearing in public in recent times. That's because she has retired since 2006 when she married a North Korean army officer. But she did make a special appearance in march of 2012 on women's day to perform her 2005 hit The Girl in the Saddle of a Steed. She was pregnant during that performence. The fact that she is no longer active since 2006 makes it even more suspicious that she was making porn with her old orchestra. Unless its a very old one. If that's the case then why is she being executed for it now ? It has at least been 7 years ! I know that porn is illegal in North Korea, but the death penalty ? And unlike the story about the uncle the North Koreans have come out and denied the execution of the ex-girlfriend.