Excellent point, Wade!
I should have clarified that the Chailly performance includes the soloist, and the "EXTRA" featuring the Welte-Mignon, is the W-M attached to a Steinway, with no soloist or anyone else visible in the Gewandhaus.
Yes, the soloist would have to make all the adjustments, if an attempt was made to accompany a Welte-Mignon piano roll.
One of the concerns I had when reading Henry-Louis de la Grange's description of the MANY variables in adjusting the W-M, was his statement that the technicians of the time, knew how to make all the adjustments to accommodate a particular pianist, the size of the room, and the idiosyncrasies of the piano being used.
Most of these technicians were killed or displaced during WWII, and it would have been very hard to find someone who was familiar with W-M's very unique requirements.
From watching the EXTRA on the manipulation of the W-M, it's one of those happy moments when one realizes that Hans-Werner Schmitz is just exactly one of those rare individuals who has been a "close friend" of the Welte-Mignon. His engineering mind knows exactly how to get out of it what its inventors intended.
Perhaps that's why this little "extra" performance by Mahler sounds, to me at least, the best I've ever heard Mahler's playing of his own music sound.