You obviously put a great deal more thought into these things than I do. The truth is that, until last night, I hadn't looked at the scores for years. I tend to just memorise things as quickly as possible and forget about notation. Wrong, I know, but the habits of a lifetime are hardly worth altering at this stage.
No, I actually have a slightly wider span than average so the Gladiolus thing could just be sloppiness on my part. I shall have to have a closer listen to Rifkin. I have his three vinyl recordings of the rags somewhere but couldn't find them last night. The only other professional recordings I have of Paragon are those of Arpin and Morath, both of whom take liberties of various kinds.
I think the whole issue is quite complicated. I notice that David Thomas Roberts, in his contemporary ragtime, also writes using this multi-part notation. His "For Kansas City", for instance, has many densely written sections with double stems, ties and beams. Listening to his playing I cannot say that I hear correspondingly different durations, only varied accents. His publisher, who actually typeset all his works, is a good friend of mine. There's nothing much about ragtime she doesn't know so I shall ask her about it.
Although I do play a large number of these works more or less intuitively, I do not feel I have the expertise to fully answer your question.