FWIW, Alvarez would make sense in the context of Spanish references in Blackmoor: The maternal grandfather of "El Cid" goes by that surname. So, "The Great Svenny" is married to "Ximena" (El Cid's wife) and rides on "Babieca" (El Cid's horse). "Rodrigo Alvarez de Astorga", finally, is the granddad of El Cid.

Not saying that this has any in-game significance: The name was likely borrowed free of any context; the Charlton Heston "Cid" movie was quite popular back in its day, and there were numerous literary treatments of Moorish Spain back in the 1960s and 1970s that Arneson might well have read.

Now, whether Arneson intended to link Uther to the family of the Great Svenny, or whether he wanted to make the Great Svenny and Uther relatives, no idea. - But one could read things this way, for sure, with a few creative liberties.