mw13
Novels are a legitimate source for historical practice and are used all the time, as in examples I provided.
You say “a practice that is several hundred years ” You do realize that given how old our religion is, “several hundred years” is a modern innovation. Kudos to you for admitting that.
BTW a point you are overlooking is that even without the Irish source the minhag is sketchy. Here is the timeline:
1- Goyim make breads marked with crosses this time of year. This is a fact it is true throughout Europe and is older than shlisel chalah
2- Keys were often shaped like crosses. Another Fact
3- Yidden started making Breads marked with keys this time of year.
The only link Alfassi provided (which may not be needed, though it seems solid to me) is that Goyim used their cross shaped keys (which nobody disputes they had) to make the cross shaped marks on their breads (which nobody disputes that they had).
Keep in mind this is not a proof at all. It is almost impossible to prove these kind of historical things. Even if we found a letter from Yankel Yatzmich describing how he loves this idea of marking breads with tzelems that he picked up in Ireland and plans to copy them by using keys, and he has a meeting with the Oheiv Yisroel where he plans to trick him into thinking it is an ancient source. It still wouldn’t prove anything! “Obviously the letter was forged” “The Apter Rav would never fall for it” All we can do is examine the evidence and reach the most likely conclusion. Even without Alfassi’s paper (which seems relatively solid to me) The conclusion that it is an adaptation of Goyish cross marked rolls (“Hot cross buns”) made this tiem of year is pretty sound