After 14 hours of research and experimentation I recovered everything. Here's how:
I used iPhone Explorer to back up all of my photos.
I jailbroke with redsn0w.
Explored the filesystem with iPhone Explorer, and tried to copy the
whole root filesystem (which, for some unknown reason, silently missed
quite a few files, but oh well).
Saw that both sms.db and all my application data were still intact.
Danced with joy.
Removed the ReleaseType -> Beta key in
/System/Library/CoreServices/SystemVersion.plist
Restarted it, set it up as a "new" iPhone, and all my sms, apps, etc
were still there. Hurrah.
Set my PC's date to a week ago.
I then backed up in iTunes, and restored to the latest beta.
Photos, notes, sms were back on there, apps were all deleted but app
data remains, so it's just a matter of re-downloading the apps.
Best Answer
TL;DR: It appears the iPhone 6 supports 24-bit audio and 48 kHz playback.
So I found a way to empirically test playback.
This first screenshot shows a recording of playback over iPhone USB into Quicktime. You can see from the spectrogram that there is data in the 22-24k range, which means that playback is at least 48 kHz.
This next screenshot shows playback over USB straight into audacity. Basically, I played -95 dB and -97 dB tones at both 16- and 24-bit audio. -97dB is below the minimum volume of 16-bit audio. While all the other tones made a sound, the 16-bit, -97 dB chunk of the recording shows no sound, while 24-bit -97dB does play a sound.