For reference, the two main motifs were taken from Pachelbel's Fantasia in D Minor. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HXfVxzeDmGA.html Right there at the beginning, you don't need to listen to all of it.
the quality of the image is much better when the notes don't move and it's our eyes and head that move to follow the notes, or the text ( like when we read text printed on the page of a book) So in my opinion these videos would work much better if the speed in which the measures pass were constant. Who cares about the red line. This videos will be watched by people who can read music, so we don't need a red line. Only that the notes played get more or less in the middle of the video frame. That's all.
Gerubach's videos are a great resource for me, as I still cannot read music very well. The red line tells me exactly where I need to look. There are many, many other videos from different channels, which show a non-rolling score. This one stands out for people like me.
It's actually an F and it's because in the stop she's using, the fifth is louder than the actual note being played; but if you listen carefully, you can hear the Bb underneath it. Interestingly, once the 16th notes start, the sound seems to "morph" so that the written note becomes the louder pitch. I'd be interested to have this explained by someone who understands organs better than I. Edit: Upon further examination, it seems that the fifth becomes louder when the note is sustained, which is why it is less noticeable in the 16th and 8th notes
I think the Allegro was inspire by Pachelbel's Fantasia in D Minor. The two main motifs, which are found throughout the whole piece, are found in Pachelbel's work: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HXfVxzeDmGA.html Right there at the beginning, you don't need to listen to all of it. This is typical Bach: The original was already nice to listen to, but he made it even better, much better actually.
They're called accelerando's and rallentando's. Bach is very rarely played in strict metronome tempo. In fact, Bach never wrote tempo marks on his handschriften, AFAIA. Even greats like Gould rarely played in strict tempo.