One of my husband's relatives was baptized as Carl but later became Charles. Charles is Karl/Carl in German. Since he was from Germany, he may have used the English version of his name.
Death certificates are not considered primary sources for information except for the death of the individual unless the information was given by the parent who should know.
Didn't the marriage license give the parents' names? Was the marriage performed by a priest or minister where there may be a church record with more information? Who were the witnesses?