If she still lived in Detroit, email the Burton Historical Collection of the main Detroit Public Library and ask them to look for her on the City of Detroit death index and if she is not there, ask if they could look in the books that have deaths in Wayne County excluding the City of Detroit. Those only cover a limited period of time but would help eliminate certain dates.http://www.detroit.lib.mi.us/burton/burton_index.htm
Since he died in 1920, did you look at her children's 1930 census records to see if she was living with any of them?
The death certificate showed St. Charles as the place of burial, removal or cremation. The following site has many of the Michigan cemeteries listed and also if they have been transcribed. Were they Evangelical Lutheran or Catholic?http://www.hal.state.mi.us/cemeteries/search.aspx
When I put in St. Charles, they listed Trinity (or what used to be called the Lutheran cemetery). You might want to call them and see if Jacob is buried there. If not, ask if they know of a St. Charles cemetery in the Detroit area. If he is buried there, see if Hannah is there.