1 - The DSATX simplys a pulse to trigger startup/shutdown. If you set the PC's powere button to send the PC into standby, then the DSATX will trigger this. LOWVOLTS is designed to shut off the PC (hard shut off) if the voltage output from the battery is below the threshhold. It doesn't simulate a press of the power button. This is more of a battery protection feature than a convenience feature.
2 - There is a variable that you can set that will turn off 5v completely. It's too early in the morning to look it up. However, you won't be able to maintain standby, as the PC needs the power draw to maintain standby state.