1) There is no power button, so the battery can't be switched off accidentally. The left button doesn't turn the battery on and off, in spite of the fact that the button has that power symbol ebossed on it. The left button has several functions...first, hold it down briefly to see what the battery level is at...the number of bars. Second, hold for three seconds to switch on the bettery. That also on the display so that you can see what the voltage setting is. It also turns on a backlight for the display. Press and hold again to switch the display battery off. Finally, with the battery/display switched on, hold down the right button for 3 seconds to cause the voltage setting to start flashing. Once voltage setting is flashing, press the right hand button repeatedly to toggle through the various voltages. At the voltage you want, press the left hand button and hold for three seconds to set the new voltage.
2) I've powered my recorders from the USB connection. My recorders require 5V dc. Both of my recorders (DR100MKII and DR2D) both run for a LOOOONG time...at a festival they both get to the 4th day before I need to worry about the battery going dead. I've never run it dead during a festival, but I think the battery would power either of my recorders for an entire four day festival if I didn't have any backup.
3) I wasn't able to verify, but I'm pretty sure that it works with two cables connected simultaneously, one on the USB and another on the output jack. I'm think I read that this is the case somewhere, either in the literature or the Naztech website.