Having another go at piecing this together.
So the story so far seems to be that Lillian Clewes first married William Harry Iredale in York. Ontario 30 Dec 1890 (as per Ontario, Canada Marriages) and he died 8 May 1897. They had the 2 children:- Grace Irene 6 Aug 1892 - 1244 W Polk St., Chicago, Illinois and William Francis 22 May 1896 Toronto.
William Harry Iredale (husband) died in Toronto. Ontario 8 May 1897.
Lillian Iredale then married Harry Gill 17 Dec 1903 in Chicago. Cook. She was married under the name Iredale
http://www.rootschat.com/links/08sp/The 1900 Census Chicago Ward 12, Cook, Illinois shows her as Lillian Curtis widowed. Unfortunately there are no death records that I know of for Illinois covering that time frame. If she married Thomas Curtis in 1899 (aahhh !!!just found that date - 22 nd May 1899 Ontario Marriages) and the 1900 Census was taken on 1st June 1900 it shortens the time frame you are looking at
There does'nt appear to be any border crossing for Lillian Curtis / Lillian Iredale / Thomas Curtis/ Thos Curtis between 22 May 1899 and 1 st June 1900 that I can see

On the 1920 Census for Chicago Ward 27, Cook. Illinois the Simpsons living with Harry and Lillian - are Lillian's daughter Grace and her husband Richard Simpson.