Do you have access to the censuses? Minnie appears on the 1900 census with Walter (54), her 4 children with Walter and her daughter Anna (25) from her first marriage. Odd part, the census shows that she had 5 children with 4 surviving. Unless these were children from her second marriage only.
The 1910 census shows Walter in a second marriage with M. Delphine. They have been married 2 years and have no children. Walter's daughter Minnie M. (28) from his first marriage lives with them.
1920 census shows Walter (73) as a widower. Son Walter E. is also listed on the same census page in another household.
The Mormon church website shows that Walter Jerome Bell married Minnie Coellen 12 January 1878 in Detroit. Says that this is an extracted record.www.familysearch.org
If you go to the Mormon church website and do a locality search for Wayne County Michigan, you can see what vital records are available on microfilm. For death records, Detroit deaths are only included up to the end of 1897. In 1898 the City of Detroit began recording their own deaths. Wayne County (except) Detroit are included to 1917.
Vital records from the State of Michigan are pretty expensive-$26.00 by US mail, more via the internet. The $26.00 includes a three year search, $12.00 for each additional year over that. http://michigan.gov/mdch/0,1607,7-132-4645---,00.html
The State website has the Genealogical Death Indexing System (GENDIS) link. You might want to look for Coellen' death. There is no soundex search and there are transcription errors.
Detroit death records are less than the State's unless they have changed their rates which the law allows them to do.http://www.ci.detroit.mi.us/health/vital/vital_m.htm
If you believe the death to have occurred in Wayne County but not Detroit, you could try ordering the record from the County which is still less than the State the last time I checked.http://www.waynecounty.com/clerk/
Here's a Wayne County website that might help.http://www.rootsweb.com/~miwayne/index.htm
If the family was Catholic, the Church records are on microfilm at the main Detroit Public LIbrary's Burton HIstorical Collection. You would have to research yourself or hire a researcher. You are allowed to search Baptisms and Marriages up to 1900 but you can look at all the death records and confirmations.