Supposedly, he recorded them precisely to demonstrate how bad the music was.
?
Supposedly?
Unless you can provide some kind of evidence to substantiate that, I have difficulty believing that he would go through the process of recording some 25 pieces simply to demonstrate "how bad the music was". Mind you, we're talking now about Mozart. That is not say that every Mozart piece is a gift from God, and should be praised sans cessation, but as far as his
piano sonatas are concerned, it is extremely difficult to argue how any of Mozart's sonatas are "bad". Compositionally, they are some of the most exceptional pieces of the "classical" form.
"Why only the minor pieces? That to me seems to bespeak of emotional immaturity"
I'm not sure what this means exactly. Citing my opinion is somehow emotionally immature? The minor pieces are emotionally immature? All of this is absolutely subjective anyway.
The minor pieces (of which he composed few) are exceedingly creative, and certainly, emotionally, closer to the nerves than many other "classical era" compositions. This is my opinion.
"I've even met people who preferred the little symphony in g over the Jupiter on account of its outward sentimentality (most definitely not for its purported depth)."
If ever you must know, I prefer the Jupiter Symphony to Symphony No. 40.
Sound and fury, signifying nothing.