Your conclusion about the 1900 census seems reasonable. It could be it reads Mrs Thomas Curl. There is no reason to believe that the children listed are not both Thomas and Maggie's since they were born after the 1880 census. Therefore Thomas probably died between 1894 to 1900. Since their last two children were born in Oklahoma that is most likely where he died. The WWI draft card for Harold shows he was born July 16, 1895 in Guthrie, OK. I would start there.
I suggest getting the death certificates and obituaries for some of the principal people you are looking for. If you get a death certificate that doesn't have the parents listed try to get the death certificate for a sibling. At least one of the death certificates for Maggie should have her maiden name if their obits don't.