Yes the ATMega8 has an ADC with 8 way multiplexer.
S-VBAT will probably be to give a shutdown when the battery voltage becomes critical. Many ESC's allow you to specify the battery type which with the voltage at startup is used to determine the shutdown threshold. Many ESC's offer a hard or soft shutdown. Not sure about S-PWR but it may be a startup check - to ensure the power is established and correct before allowing the software to start.
Originally Posted by George Shering
For 2, a possibility. Does the ATMEGA8 have a built in ADC?
If not then S_VBAT and S_PWR may be the voltage measuring circuit, checking how long S_VBAT takes to get to S_PWR after being released.