OK, update but not much. NO macaroni salad in any of my old cookbooks, latest 1906, then I have a gap till the 1950's and plenty of it.
Delmonico's and the Waldorf-Astoria pushed salads of all types in the 1880' and 1890's, Waldorf Salad for example.
Macaroni Salad is basicly potato salad with macaroni instead of potatoes. (Yes the e is in many old cookbooks) Macaroni was imported till about the time of WWI for several reasons, so it would be logical to use it instead of potatoes in a Gourmet eatin' house like Delmonico's. Also till 1913 and Helmans coming on the market, mayo had to be hand made, also makin' it gourmet.
I will continue to search, but the Delmonico's still might be the source.