APU BAT OVHT light is on the aft overhead panel. See
http://www.b737.org.uk/electrics.htm
The 'Buffet Alert' scratchpad message means "Current conditions result in a maneuver margin less than specified."
Alternate nav system is purely IRS based. See
http://www.b737.org.uk/navigation.htm
There are a few good websites and books out there about the 737, if your company is not providing you with the material (FCOMs) to answer these questions then perhaps you should be finding & bookmarking them.