I studied in Rome in college and took a course on the medieval history of the city. My prof who was quite liberal and far from a Catholic even noted that Pope Joan is a load of BS. No serious historian actually believes Pope Joan existed.
As for the mosaic in St Praxede, do you have any more information about it? I noticed the halo is square, which means the person depicted was living when the mosaic was made. The same church has an image of the current pope, Pascal II, with a square halo as well. Are there any records of a prominent Theodora from the 9th century? Could she possibly have been a bishop's wife - hence the term episcopa?
oops, that post should have been to eastsider
"Episcopa" above her image refers to the fact that Theodora was the mother of Pope St. Paschal I (817-824), who had the Church built to honor her. Dissident Catholics who clamor for women's ordination have foolishly tried to coopt the reference and spin it as her being a bishop.