Mathilda shows up on the passenger list in 1867 along with the rest of Stephen's family; thus, my presumption that she is/was Stephen's child. Is there a known connection between the two? Maybe she is related to them or they too have a daughter named Mathilda.
From her German burial record - there is one brother and two sisters still living if I recall correctly - do you know who they were? My tack on this was - let's see if her siblings' records reveal a birth town - don't just limit to Mathilda's available records. Do you have any info on her siblings? Is there an English obit in a local newspaper? What about obits in local German newspapers of the time? This is likely where they would mention exactly where she is from. Does her death certificate mention her mother's name?
I do not have access to the records - are they on-line? I know the parish has an on-line presence though but typically catholic church records are not on-line in America.