This is a story, in conventional sense, not the etymological "Narration/ Recital of True Events". But this happens when one tries to make a "True Story" on some one still living - or some one dead so long back that the life is shrouded in mystery. Any story naturally have to have TRP, and especially in case of living person, people have to be doubly careful, not to hurt the feelings, and suppress/ modify truth to the audience and the person's choice. This seem to be a glaring example of that.
As a truth or biographical sense, based on the information I could access, it ranks somewhere around 1 out of 10, but on entertainment value, it is 7.
The only thing that seem to be true in it, is the name of the male protagonist, Howard, and links to his musical numbers, except the name-sake "I wonder Who Is..." which he had appropriated from other (Harold Orlob) and if there was one, may be...
But except Howard, the other protagonists, thank fully the names (screen) don't ring bell - I Wonder who were they ?
Katie could have been his second wife Ida Emerson - the ages almost match (3-4 years younger), but who was on screen Mabel Barrison, on whose tour he had tasted first success ? Lulu ? Doesn't seem to be based on Mabel's biography - since she did marry Howard in 1906 - and stayed married till her death in 1912 of TB. Second though already successful on Vaudeville, she was much younger than either Joseph or Ida - in fact by age standard she would have been Katie (Joseph-12 to 15) - only that here there was no child-hood romance - she was Canadian. Of course if one goes by the timeline of I wonder.... (1909).... it would be she.
With so many contradiction, I will remove the claimed "True" and "Story" and consider it to be a fiction with no relation to any one (then) living or (now) dead. But once that is removed, probably, it is any other Musical Romance. Enjoyable, thanks to the musical number, but nothing too out of the way as far as the plot is concerned - now HAD it been really true... !