I can speak to this octave business, since I've never had much problem with any octave passages.
For me, it came about as a kid... 15, I think... when I became absolutely obsessed with Horowitz's Hungarian Rhapsody #6. I even timed the sections with a stop watch and practiced until I matched his speed! LOL
As most pianists do -- and wrongly -- is I made a hell of a lot of effort, hitting the keys, using a lot of arm, wrist, etc.
Eventually, I'd simply wear myself out, have tired or somewhat hurt muscles, I'd recuperate, and do the same thing HARDER!
After a number of cycles of this nonsense, it dawned on me that Horowitz was not banging the hell out of the piano, and that the octaves had a melody, and that the top notes were more pronounced than the bottom.
So I played the octave part with both hands as I'd really like it to sound, a melody with musical elements -- duh! -- and this changed everything.
An octave was a melody with a lower note as the accompaniment -- in the case of HR6 -- and so I abandoned the muscular hitting approach, and went back to fingers.
FINGERS on the key, supplemented by wrist, forearm and upper arm, played so the effort was very easy and very relaxed and very minimal.
Its kind of a vibration which starts on the key pulling the key with fingers and necessary assistance from higher in the playing "chain" of piano playing muscles.
In fact, when playing or practicing the HR6, I do not even allow the key to come all the way up so as to decrease the distance I need to depress the key, and it can't be much more than 3/8ths of an inch.
Of course when more volume is required, it takes more effort, but you can get quite a big sound ON THE KEY.
And that's the secret... tiny movement on the key adding or subtracting necessary playing elements for volume/sound desired.
HR6, BTW, is different levels of both speed and volume, and its not until the last page that you need to really ramp it up, and even then that is relative to what is played before.
Horowitz does not bang through the whole thing, because its not realistic or necessary.
He starts piano, and "terraces" the volume up by degrees. It is not but the last few chords that are crashing, after all the "real octave work" is done.
The Funerailles is also great for learning this technique -- primarily left hand, which I also was obsessed with!LOL
To some up, I do not think of octaves as "octaves", but as a melody with and accompaniment i.e. the lower note of the octave, and distribute the "work" among all playing parts, fingers setting the structure for everything else.
IOW, octaves should be "music".