My car must be set to "auto update"
There is no such thing as "auto update". Someone has to give it a go on the app or in the car. The only exception is that in certain rare cases in the past when a car's software was a way too old (years old) due to the owner refusing to apply numerous updates then Tesla had forced the updates with no user consent.
... and I still want to know why I can't cancel an update remotely ...
This is a good question.