The answers can be found in this thread.
1) Is the battery user replaceable? Where can we get replacements?
Yes, it uses Molex connection so it should be easy to replace the battery, you can get the replacement from HeadAmp
2) Is the analog signal path completely isolated from the digital side?
The charging board is completely separated from the Amp/DAC board.
3) Is the DAC upsampling only (44/16 in or whatever) or can I use it to listen to 24/88 and 24/96 sources without downconversion?
I'll let others answer this.
4) Are general specs and info available in one place?
http://www.head-fi.org/forums/f38/he...ml#post3376651
5) Why no SPDIF input? That is sometimes easier to deal with and more serious than USB. Do you think you might add it or make a new model? I don't want a full sized home unit, but a portable unit larger than the Pico would be fine.
It would make Pico bigger, a Pico should be small
and there are alternatives out there.