Lf
taamei minhagim is a more recent source than the ohev yisroel, not sure how that helps. That said id love an earlier source please dont give up. I bet that alfassi fellow would love one too.
Dy
You really don’t consider that a source? They (allegedly) used keys to make cross/key shaped marks in their bread at this time of year. And we make key shaped marks or key shaped bread which evolved to sticking a key in regular bread.
Its impossible to prove of course. But isnt that suspicious? Wouldnt you call that a parallel?