> don't know where the i [in Hamelin] comes from in the English transliteration

Could just be that it’s a very inconvenient consonant cluster (and and a speaker of modern English will to some degree turn it into a [lən] or [lɪn], however you spell it).

I’m an English speaker and when I saw it written “Hameln” I thought it was a typo.

It comes from the same place as the i in Munich.