I think this is up to interpretation. I only bring out the inner fingers. Basically the pinky and thumb are whisper quiet, but still audible.
Yeah I guess it's up to personal sense. For me, for the RH tremolos, I try to bring out the lower notes, with the high E (or high C# in other passages) very quiet; I like hearing the harmony between the two lower notes, so I try to play them at the same volume, with the middle note perhaps slightly louder (only because it's the higher note of the two) but not by much.
For me separating hand position and voicing causes problems. If I do them separately, I end up with hand positions that enable me to hit the notes very easily, but with fingerings that make it uncomfortable to voice the way I want.
I guess for me, I tend to learn the notes first and then do the voicing later for a piece such as this one, so that I can get my head out of the sheet music quickly and get a better feel for the flow of the music (to know what voicing I want to do). For the most part I think for a good portion of this piece it doesn't matter very much -- the proper hand positions are fairly intuitive -- but I have trouble with certain passages such as this one due to the stretch that's needed.
I think you are lifting your 2nd finger and especially your 3rd finger way too much. Some pianists lift a lot and can play forever no problem. I'm not one of them. I could play the passage we're discussing in tempo if I lifted as much as you or even more than you, but that would probably cause me strain and affect my playing for a few seconds afterwards. I guess there are a few seconds for recovery as you play the arpeggiated chords before you do the scales...
Oh for that, well a lot of the lift is actually because of the forearm rotation and not the finger itself -- it's just along for the ride. I could do that for quite some time without getting tired; in fact, it's how I do the tremolos in the other passages to get the reach. Although looking at the video, it looks like I do straighten my index finger unnecessarily; I was probably trying to emphasize the middle note since it kept sounding soft. The main issue I was having was that my hand would get tired doing the (E C#) with the high E, but I think using a (1 3) instead of (1 4) fingering has cleared that up.
Out of curiosity, have you tried playing from the Friska to the end in tempo? Also, are you doing a cadenza?
Hmm well I'm not sure what you mean by "in tempo"...do you mean at performance speed or at a consistent (slower) speed? I'm still in the process of learning the notes so I don't have it at performance speed yet. However, I can play through the parts that I've learned at a pretty consistent speed. As for what speed it is, I don't know exactly but I know I play through the 3 octave scales in about 1.6 seconds, which means roughly 110 beats per minute. So I can play it at that speed more or less. I'm learning the piece from the end forwards so I actually haven't learned the early sections of the Friska yet, but what I know, I can play to the end at a consistent tempo.
I haven't decided yet on whether or not I'll do a cadenza. There are a few sections that I'm modifying though, and I'm likely revamping the entire section after the scales and before the ending octaves, because I like Horowitz's treatment of it more (mainly, recapping the different melodies). Otherwise, what I'll more likely be doing is modifying some of the passages to suit my liking more; a cadenza will likely wait until after I've learned the whole piece.
Anyway, so after practicing a bit, I've attached what it sounds like now. Unfortunately I was somewhat distracted by trying to look at the camera screen and directly at my right hand so it was somewhat more uneven than without trying to record it (since I hold the camera with my left hand while playing with my right). I think my pinky is staying on the high E more than is necessary, but I'm not sure if that needs to change or not, since it's softer now.