Following the timeline:
28DEC1879: Viktor Emanuel b in Sweden
09AUG1880: Hulda's birth record gives her father's surname as "Backlund", so perhaps that answers my question.
13MAR1893: Viktor Emanuel arrives at NYC on the "Island" (destination: Worcester MA)
1900: In census Victor under surname "Swan" with his family (his mother's maiden name sometimes given as "Svahn" for some reason; her husband had just died)
05APR1902; Wilhelm Emanuel Johnson marr Hulda Sophia Backlund in Brooklyn
12&19APR1903: Birth & death records of infant son, George, (in Worcester MA) lists parents as Wilhelm E Johnson & Hulda S Backlund
31OCT1905: Axel Emanuel, born to Wilhelm and Hulda in Brooklyn (from b cert)
1910 census: "William", b ~1880, Sweden, helper, steel mill; to US 1890
1915: "William", b JAN1872, Sweden, Steel wkr
12SEP1918: Victor Emanuel Johnson, WWI draft regn: b 28DEC1879, Citizen of Sweden, res Burlington Co, NJ
1920 Census: "Victor", furnace tender, wire works
1930: "Victor"; heater, steel works
1942: Victor Emanuel Johnson WWII draft regn, Florence, NJ; b 28DEC1879, Sweden (tells exactly where); references Clarence Johnson [his son]
So we definitely have "William"/"Wilhelm" between abt 1901 and 1917, but he's the same age as Victor, has the same middle name, came from Sweden around the same time, also works in the steel indusrty, has the same surname. Plus, kind of like Superman and Clark Kent, they never exist at the same time, and one appears when the other disappears. Sounds to me like "Victor" just found "William" to be a more "American" name, and used it for a time.