I think there's a good chance Evans actually did it...

Going back to the possibility of seaman in Mrs Evans, the pathologist, Proffessor Teague? wrote in his report that there appeared to be some vaginal bruising, but that the case was disterbing enough, and he would not persue the possibilty that she had been raped. A swab was not taken. Christie did have sex with his victims, he said so in his confession. He just had to kill them first.

As for false confessions, people make these all the time. Stephan Kisko confessed to the murder of Lesley Moleseed, a ten year old child, and served 16 years before being cleared by DNA. The real murderer is now in prison. The Cardiff three confessed to the murder of a woman, and served many years before the real murderer, who left his blood at the scene, was caught amd convicted. The Guilford four, Margaret Livsey, perhaps now Amanda Knox, the list goes on. Confessions mean nothing, unless they are made in court.

There's certainly room for doubt. I'd hardly describe Christie and Evans as 'co-habiting' but whether either of them did both murders or they did one each, some kind of collusion or mutual knowledge is highly likely. The film is in my view simply a dramatisation of Kennedy's book, the 'ofiicial version', rather than a definitive explanation. If this case had been solved without doubt, it might well have faded into memory and wouldn't be discussed today.
As a society, we seem to appear to be skeptical but then accept 'official versions' as fact. So, officially why did the U.S. get involved in Vietnam?? It's since been admitted (it's on record) that the Gulf of Tonkin incident was a total lie which resulted in thousands of American deaths and a million Vietnamese casualties. And JFK? (he wanted to shut down the private federal reserve and not get involved in Vietnam) 9/11? 7/7 in London?? the list goes on. Anything that involves politics involves money which involves agenda.
The British government had its own agenda in this case, which was not to admit that Evans was innocent even after Christie's other crimes had been exposed, so they only gave Scott Henderson 7 days! to do a full review. Evans was only pardoned in 1966 for the murder of baby Jeraldine (apparently this is the correct spelling of her name) which was the murder he was tried for but was still held accountable for the murder of his wife, which he hadn;t been tried for. Therefore, they were able to pardon him without actually admitting they'd hung an innocent man, which was quite canny on their part. In 2004, Evans's sister tried agin to get a full pardon but was turned down, although it was acknowledged that Evans was 'probably' an innocent man.
If push comes to shove, i would say they both had a hand in the Evans murders.