The Germanic name Mathalwin is definitely attested and existed in medieval times in Germany (I have no data on Old English nor Old Norse at my hands, it is not in the Domesday book and all names starting in Madal- currently on dmnes.org are Germanic names from today's
France). However, some connection to
Melvin must be made (either by continuous use and survival in some local pockets, by being the name of a saint, or by revival in a literary work). We need to find the missing link or we are stuck here.
And than, there is the unrelated name
Malvina introduced by
James MacPherson. And another unrelated, but similar sounding name is the Spanish term for the Falkland Islands: Islas Malvinas. The archipelago is named after the French harbour St. Malo.
--elbowin