Regarding Mary's marriage to john F. Frady it probably was annulled or didn't take place. I don't think they could have been divorced in time for him to marry Stella Sprague.
My sister has a marriage license in Chelan County, Washington but the wedding was called off three days prior. If you just look at the records, you'd say she married Mr. Matthews but she didn't.
Bds in City Directories usually means boards. That she was boarding at the address shown.
She probably did die in Iowa City, JOhnson, IA. Death certificates weren't mandatory in Iowa until 1920. However Johnson County Recorder has some deaths starting in 1880.
I'd contact the Johnson County Historical Society, www.jchsiowa.org/collections-research/
. They may have old obituaries and inventoried cemetery records.
The Iowa City Genealogical Society, www.iagenweb.org/johnson/ICGSmemberData.htm
might also be a resource. Successful Searching!