Sounds like the TV show didn't want to get into the complexity of the siege at Riverrun -- in fact, the TV show never does say what happened to Edmure or the Blackfish, do they? Both left the wedding hall before the carnage, but I don't think they ever referenced either one again, as far as I can recall.
Jamie going to Dorne is interesting, but my sense is that they needed a plot element to get Jamie out of King's Landing so that Cersei would be alone when the **** hits the fan. And it looks like they're setting it up so that Lancel is the one who turns in Cersei for screwing him and for her role in Robert's death. The question is if the TV show will go the full nine yards and have her sleep with Osney Kettleblack to get him to set up Margaery for adultery as well. They certainly could at this point, as they are doing a good job portraying Cersei's anger and jealousy towards her, but I can't remember if Osney is a character in the TV show.
One of my favorite scenes in the books is when Cersei is led to the room where they are torturing Osney, suddenly realizes she's ed, and tries to run away.