Why is it not "....qu'elle s'agit....." as both the sculpture (which we know is a woman and is specific) is feminine and/or Catherine is feminine?
Also is the use of the conditional perfect, because this is a legend and therefore there is some doubt as to whether it actually happened?
Martin, "il s’agit (de) " is an invariable impersonal verb - it only has conjugation in the 3rd person singular.
https://www.lawlessfrench.com/grammar/impersonal-verbs
https://www.wordreference.com/conj/frverbs.aspx?v=s%27agir
And to answer the 2nd part of your question, yes you are correct, because it was a legend it "allegedly happened" or "was reported to have happened" which is one of the uses of this tense in French, especially in journalism.
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