Hello Luke!!
thanks so much for sending this, I'll try to hear it later! Exting!
Haven;t told you the whole story, so here it goes: I don't remember much about the lessons I had when preparing this sonata, it was a very strange/difficult time back then, and that piece, and the others of that period are somehow hidden behind a curtain. However I went to a competition later that year. Didn't win, but played alright. One jury member who had been in my side came to talk to me too after, and gave me her advice on what I had played. I remember her talking about the tempi in my variations

And how she thought that despite musically, I had taken perhaps too many liberties!

That discussion has stuck in my head, as she had soemthing special in her! (so, i didn't play with a steady tempo then...!)
I had a look at them again after you asked, and all these years later, I agree that there should be consistency between them.. When there "have to be" tempo changes the composer writes them, adding another character or tempo specification before that particular variation.
Sometimes I feel the theme is played a touch slow, and that might maybe push for the changes needed later...

However, you have someone as knowledgeble as Andras Schiff backing up the tempo changes with speed, so it is up to you what convinces you most as an artist in order to express the piece better. Have you tried to record yourself and listen after? No worries about notes etc if it's new, but just have a listen from outside how the "story" of the piece is going.
Sorry I cannot give you a more in depth explanation...! Taking decisions is a taugh call to make

But is the most fun! And they can change over time. And you should be in the right path, the kwyword being "subtle"
Maite