HEY! PHONE OEMS (yes, including Apple)
Can we just jump to the end of this long, boring button saga?
Remove the volume buttons entirely and these stupid single function buttons, and replace them with a single capacitive slider. Then you can change volume by sliding your finger along the strip and tap to say take a picture. In fact, you can divide the slider into two regions and have TWO programmable "buttons".
"But I'll be changing the volume by accident when I brush it" Because you don't accidentally change volume by leaning on the existing physical buttons? So tap drag to change volume. Then you can add say double-tap drag to change zoom. And so on.
BTW, this also makes it easier to make the phone waterproof, uses fewer parts and makes cases easier too.
You're welcome.