If the Church’s teaching is true, she does not need to support it by treating fables as facts - to treat fiction as fact would be the very worst kind of argument.
We shouldn’t confuse “fiction” with oral traditions that do not necessarily have the benefit of solid historical support.
“Fiction” implies something made up of whole cloth. Oral traditions develop over time, and are believed by many. Some tradtions are confirmed by the archaeological record, like the location of St. Peter’s tomb underneath his basilica. Others lack such support, and are even contradicted by the evidence.
However, even if there is no official support, there is definitely no harm in believing such traditions, if they are not shown to be false. People have been believing in in them for centuries. I think it would be an insult to tell them they believe in mere “fables.”
Just as people in court are innocent until proven guilty, the generous and Catholic mind should, I think, assume these “small-t” traditions as true or at least worth believing in until clearly shown otherwise.
As for calling St. Peter the first pope, there is certainly no harm in that, if by “pope” we do not mean a certain title which has developed over several centuries, but by the office, in particular the pope of Rome’s teaching office as pastor of all Christians. Any good Catholic would assume that St. Peter held this office without, of course, assuming the particular title. A rose by any other name would smell just as sweet, after all.