I'm not sure what to really think in this case. It does seem like Rivera likely did it acting alone.
However, our Justice System doesn't like to be wrong. There are examples all over the place of the Justice System deciding someone is guilty and ignoring other evidence at the time. Its not super common, and usually the face value of something holds true, but not always.
I sure hope the State is looking into the allegations from Rivera seriously. It seems outlandish, but many of the biggest crimes are often unbelievable, outlandish things.
It reminds me a lot of a true crime podcast I listened to recently. A man and his girlfriend had a home invasion occur, they were both tied up, the girlfriend was kidnapped and the man instructed to obtain money for her release and not go to the police. Later the next day the man called the police anyways and reported it. The police immediately assumed he murdered her and was spinning some elaborate story. They held him for days while the girlfriend was held hostage and raped. Eventually she was released, but officials didn't believe her story either. Eventually the guy that did the home invasion attempted another one and was caught, which ended up confirming the couples story and clearing there names.
Again, the above isn't the standard, but extreme things do happen at times, and it'd be a disservice to not treat those allegations seriously if they did lead to a larger crime.
FYI, if interested in the podcast, here's a link, its a two-parter. https://thisiscriminal.com/episode-167-48-hours-6-18-21
You've pretty well captured my thoughts as well. I think the most plausible explanation is that he did it. But I also think that once law enforcement creates a theory they aren't very amenable to alternative theories. That's especially true in a high profile case where there is pressure to arrest and convict.
So do I think they probably did their diligence to investigate these new allegations? Probably not.