The couple also had two additional daughters that you do not have listed. Daughters Jane and Jennette/Janet. The girls were not living in the 1880 census home of their parents. One was living in a boarding house as a servant/maid (Jane), the other daughter I did not look up. Notice the 1900 census gives Margaret (Campbell)Craig as having born 5 children with 5 still living.
Both daughters are listed in IL Death Records with parents John Craig/Margaret Campbell: Jane (Craig) Baxter 30 Sept 1865, died 7 Feb 1937. Jane married George Baxter 31 June 1884 in neighboring Grundy Co IL.
Second daughter is Janet (Craig) Steele, 2 July 1863, died 22 Sept 1939. She married John Steele in Will Co IL 21 Sept 1883.
Both girls lived at least from 1900 on until their deaths in Bureau Co IL in both the Hall and Spring Valley area.
As far as Margaret's obit, I'm too far away from there this time of year to stop and look on the films for an obit, but you might try the Bureau Co Genealogy Society for a request (the LaSalle/Peru area papers may have also carried a notice). They may also have a better/more complete reading of the Miller Cemetery in their holdings. Also you could try the Bureau County Courthouse for a probate record for either/or John Craig or his wife which might give you better idea of death info for John Craig.