I have real trouble understanding how anyone could believe this a story like this one. First off the stoning death of 'Pope Joan' after giving birth during a public processional... Give me a break. We're supposed to think that she could not have come up with some excuse to be indisposed rather than being out on parade? It is one of those details that just scream "this is a legend". "Yeah, and then she had a baby when she was being carried around in her pope thingy. Then a mob killed her." "Whoa, gnarly." And of course the stories disagree on if she was in fact killed or even what she was doing when she had her son. (Who was either killed or went on to be a bishop.)
Next up the infamous marble throne, the sedia stercoraria, really was a throne of sorts. It was a Roman toilet for use with a chamber pot. Only a fool would fail to realize what this was used for. Or someone with very little historical (or practical) knowledge. If this is a big secret thing why would they have a chair installed? Why not a quiet disrobing back in the Sistine Chapel after his election?
Very compelling is the fact that the story didn't circulate before the 13th century. Was this real history one would expect some account a little closer than four centuries after the fact. Stories like this, or Jefferson having relations with a slave, are written about at the time. They do not just pop out of no where centuries later unless they are made up. So why are their shrines to her if she didn’t exist? Why do we put up plaques to Sherlock Holmes? Will someone think he was a real person in centuries hence just because there are cast bronzes of him in Baker Street?
In addition there is the inconvenient records of Pope Benedict III, the actual Vicar of Christ at the time. Everything from minted coins to paper records shows only Benedict III, there is not a Pope John VIII until AD 872. Nor is the fact that Martin of Polonus, an important historian, wrote about her in 1265 at all compelling. He is just repeating an urban legend he heard. Franklin D. Roosevelt, yes the president of the US, wrote about Sherlock Holmes as if he were real. Does that mean he was? Or because any of the hundreds of other noted scholars from our time have done the same?
Lastly there is the supposed credentials of Mr. Peter Stanford himself. He is the former editor of the Catholic Herald in London; does this prove that he would be inclined to be friendly to the church? No more than it would prove that I, a former Catholic, would be biased in favor the Vatican. Nor is being Catholic proof against believing nutty things, I think rather the opposite is true.
I'm no friend of the Roman Catholic Church, but this story belongs in the same category as the Protocols of the Elders of Zion. Amusing if you don't take it seriously and she was definitely not real.