Hell never existed in the DBZ manga, that was an anime thing until Super came with the "canon hell" therefore, Frieza never really watched Goku vs. Kid Buu cause he was trapped inside a cocoon in the middle of nowhere with cheerful angels and animals.
Yep, the last time we see Freeza in the manga is when Cell shows us what he was made off
So? DBK lefts some filler, the DBZK FC even lefts the Videl fight againts the Nappa lookalike (which was 100% filler, and was cut in the japanese Buu saga Kai)
Kai is not technically filler-less, it lefts filler and even cuts canon material in the saiyan saga
So the vertict is, Freeza never had alzheimer, he just never saw Kid Buu, the anime messed it up (just like when Vegeta "knew" that Freeza destroyed planet Vegeta priod to Raditz trip to Earth, despite the fact he acted shocked when Dodoria told him what really happened to Vegeta) and yeah they tend to do that, so you guys better read the manga
The same thing can said about Pan beating Goten in the TB, sure Pan may have Gohan's mystic powers, but overall, i doubt she could beat her uncle