Hi All! I'm the original poster of this thread, and I'm back with an update - and some good news:
With a few months of work, I've managed to "stretch" my hands to be able to play an octave with hands above the keys (literally just barely in terms of reach but with decently consistent accuracy) and quite comfortably at the edge of the keys.
I wanted to post some updated pics here in order to give hope to other small-handed pianists out there who may be struggling with the same problems. (We need support groups for the small-handed pianist, truly.)


If you compare these with the first set of pics I posted a few months ago (see first post in this thread) where I couldn't play octaves above the keys at all without depressing other keys in the middle and couldn't play at the edges comfortably, you can actually see a difference.
Here's what I've been doing that has led to being able to play octaves more comfortable plus an actual slight increase in span:
1.
Extremely careful daily stretching (I cannot emphasize the word "careful" enough. No increase in span is worth an injury that would lead to being unable to play at all.)
2.
Learning to play octaves with no tension via relaxed arm weight and "bouncing" motions: Michael_C and PianoPlayer002's advice was spot on. (Thank you!) I started working with a teacher again about 2 months ago, and she had me doing these "drop exercises" starting with smaller chords and eventually to octaves in order to be able to play loud and consecutive octaves with arm weight and no tension. We also practiced a lot of "bouncing" and "rebound" motions to move between octaves. (Previously, I couldn't achieve great volume over a long duration for octaves without massively tensing up or hitting incorrect notes due to small hand size.) In addition to practicing the concept of the "relaxed drop" when playing octaves on the piano itself, she had me practice this drop motion in my daily life (ex: dropping my arm on a pillow repeatedly while not at a piano). I must have done this thousands of times if not more over the course of 2 months. For those of you who have the same anatomical problems, keep practicing this.... it really works and will do wonders for your octave playing. I nearly cried from sheer relief the day I realized I was finally able to play octaves at max volume from above the keys with little or no tension consistently when my teacher said "good job" (after a lifetime of struggling with Small-Handed Pianist Syndrome and months of grueling, daily work on all of the above).
3.
Deliberately tackling pieces that have lots of octaves. Call me a masochist or whatever, but I figured I may as well tackle this head on... so I've deliberately been practicing daily with pieces like the following, which have helped:
--Chopin's Opus 10 No 12 Etude (all right hand octaves)
--Schumann's Papillons (the pieces themselves aren't difficult and don't require Very Loud octaves, so this is an easy way to get warmed up in a relaxed way)
--*very slow* practice of various octave passages from Liszt like Mazeppa, Hungarian Rhapsody, the end of La Campanella, etc. (the reason being that I'm trying to get used to playing consecutive and/or loud octaves in different scenarios - ie: don't want my hands to fall into a habit of playing octaves *only* for the note patterns in the above)
--*slow* chromatic octave scales (I've found it's important to do this slowly in order to stay relaxed and not build bad tension habits).
It's amazing how the hand does eventually get used to this until it becomes almost second nature, and it's possible this may have contributed ever-so-slightly to the stretch in span I've been able to get thus far.
Also, I think it's about training your body to recognize the right "angles" required to hit the octave from above until you can do this via muscle memory. (This angle differs depending on where on the scale the octave is and whether it's black or white keys, etc... so it's important to train yourself to be able to do this for a variety of octave notes.)
The challenge with small hands is that if I'm off by even a millimeter... you hear dissonance. There is literally zero room for error when landing octaves for the small-handed, so it unfortunately takes us probably 100x more repetitive practice to get an octave-filled virtuoso piece down without imperfections vs. the rest of the population. It necessitates consistent accuracy down to 1-2 millimeters or fractions of millimeters (hopefully more for you if your hand is bigger than mine)....which means lots of patience, determination, and hours of repetitive practice.
4.
Adjusting my bench to sit higher has also helped
5. Playing octaves with my
wrists slightly higher has also helped
I still have a ways to go and am somewhat hopeful (but not delusional) that my hands my stretch a bit more. Longer passages (ie: many measures) of fast or loud consecutive octaves are still a work in progress as far as accuracy, speed, and keeping it tension-free goes. I don't think I'll ever be able to play a 9th above the keys comfortable (though I guess you never know), but I think (hope?) that being able to play an octave above the keys with little/no tension opens up most of the repertoire....since it means being able to play octaves at max volume or consecutive octave scales without as much in/out motion, and every chord above an octave can more or be rolled, rearranged, redistributed, or occasionally played an a dropped note. I'm still trying to figure out how to play the opening of Rach 2 in a way that doesn't sound weird with quickly rolled chords, but hey it's a start. I'm also just an amateur, so hopefully no purists will be too outraged if I end up having to take some of the necessary liberties with scores that I've just described if I'm ever fortunate enough to have the chance to perform these in public someday.
My main point:
Don't give up, fellow small-handed pianists! You would be surprised what you can do if you persevere. (....though do it safely; don't pull a Schumann or worse.)
Also, your hands *may* expand slightly even after you think you are done growing. (Hey, I'm in my 30s and have been playing since I was 5 - though to be fair, only 12 of those years was with a teacher.)
Thanks so much to everyone for the friendly, helpful tips thus far. What a great community this board is. Hope this helps encourage the other small-handed folks out there.