Kiberd talks about exactly this point. He says that in June 1904, Joyce had an encounter with a man like Bloom, the details of which are largely unrecorded."It is as if Joyce were seeking to recapture that passing moment and asking how it might have developed, before that other major event of June 1904 took over his life."
He goes on to say that Ulysses is a book obsessed with missed or insufficiently developed encounters. He says that it is "trying to restore to lost moments of history a sense of the multiple possibilities that might have flowed from them, before a single subsequent event took on the look of inevitability."
I think it's in this context of multiplicity that it's helpful to to notice Stephen Dedalus's observation that, as Kiberd paraphrases it, "a man of genius sees every error as a portal of a new discovery, an aid to the understanding of the world. We go wrong, but only in order to go right."
A wise maxim, I think for anyone involved in reading Finnegans Wake.