1. The thing that drains battery is the always on mobile data option, go to mobile network settings and untick "Enable always-on mobile data". Another thing you can do is adjust the Auto Sync Options for your accounts like FB, Twitter etc. If you don't care about FB or Twitter updates turn everything off and only keep the "Update when I view/Open Widget/APplication" turned on. Anyway, the screen drains alot of battery so if you're on it a lot then its rather normal for a fast drain. Lower your brightness and do not use auto brightness. I am assuming your phone is factory version and is unmodified right?

2. When you turn on GPS or when you are using navigation application like Locations?

3. For Viber, I've never had force closes before though, so i'm not sure whats wrong with yours, but simplest option at the moment is to just uninstall it. If your phone is rooted, you may use Titanium Backup to freeze that application.

4. There's quite a few on the market, personally I'm using Freenote, a lot of options and its free =D

5. Extended batteries as in Powerbank?, both are reliable so no problems there, of course if you want more charges out of it get the one with the higher MA.