I have the same questions about Betty and where the children were.
As far as she being listed under her Parents as single but was actually married, I have found the same situation in my research with both my family as well as my wife's family. I found about 3 or 4 cases where this occurred in the 1900 Census.
Sorry I could not help with your questions but I do have more information about Gladys's family such as her mother's background, some info on Joseph's background as well as Gladys's siblings.