Some interesting content here!

I would say that the first orchestrated backing instrument was an accordion, or perhaps the player pianos and band organs which date to the late 19th century, and could be supplemented by another keyboard to play the "live" melody part.

In the 20th century IMO, the predecessor of all modern electronic organs was the Hammond S-6 chord organ, which had some facilities for automating the bass (root and 5th pedals), and rhythm accompaniment (a "rhythm bar" which you could operate with your wrist while your fingers were holding down a chord button.)
The chord and bass tones could be orchestrated by "tablets" which changed the characteristics of the voices. Pretty advanced for 1950, and fully electronic with valves (tubes), not mechanical tonewheels like Hammond's other products.

Another thing the S-6 had in common with later home organs and portable keyboards, is that it was marketed toward the home hobbyist with a mild interest in music, rather than trained professionals (who would certainly have preferred two manuals and a full pedalboard.)

I believe that John Lennon composed at least one Beatles tune on, or for the Hammond Chord organ. Some good stuff on this thread!