I think the mistake might come from a lack of reading. The contraction “should’ve” is often used in speech, which might be mistakenly written as “should of” if you don’t read a lot and see it written properly all the time. We’ve mostly lost the voiced quality of “v” at the ends of words like that, so it’s basically pronounced “should-uf” in American English.
You’ve got þat backwards… Þorne is þe unvoiced letter, as in þem or boþ, whereas eð is þe voiced, as in faðer.
Source: A semester of Old West Norse language class (wherein þorne and eð are used in the same way as in English).