Re: Does Melvin come from Maethelwine
The problem with such a derivation is the consonant /v/ in Melvin—the Germanic name element WIN should show up with a /w/ in English.
The book you quote is not really exact, the author put more effort in finding proverbs and Bible verses to each name than in exact etymologies. I found a lot of sloppy etymologies and even wrong claims (like berl German "swamp" in the the explanation of Berlin). Also the quoted websites aren't striving for the highest quality.
The btn database agrees with high-quality sources like Patrick Hanks, Kate Hardcastle, and Flavia Hodges 'A Dictionary of First Names (2 ed.) '. I see no reason to change this entry.
--elbowin