He asked Fitzhagen to make any necessary changes to the cello part.
The cello part is very much subordinated, usually just doubling the bass line in the piano.
Listeners had to take the triplets in the cello part largely on faith.
Composers of the Classical music period started writing cello parts which had a life of their own.
He also occasionally joined the quartet as a performer, which made him need to add an additional cello part to his music.
Among the passages new to the string quartet are measures 17-24, 125-126, and a cello part beginning at measure 63.
These parts do not double the cello part.
Mr. Harnoncourt - you can probably guess - had never seen the score or the cello parts.
The piccolo cello part was likely a later addition; the part is high and quite technically difficult.
The cello part does not dominate but fades in and out of earshot.