It's very normal. Switch to WiFi and you'll find it draining a lot slower. It's normal, for all phones perhaps. Depends on your usage too. If just standby, let fb twitter or viber those do some data transfer from time to time, it lasts. If you keep on surfing or download(data transferring), it drains hell a lot faster.
No setting for it. I got no idea why it isn't there. Supposed there are two sensors: proximity and light.
I'm still on stock rom but rooted and downclocked. Now normally data on, 12 hours take me around 50%. I think is quite good. Sometimes last even longer.
GSM (anything below 3G) WCDMA (anything above or equal to 3G *at least in our phone la~*)
GSM--> GPRS(56k)/EDGE(236k)
WCDMA--> 3G(384k)/HSDPA(14.4m for W)
Confirm tell you no auto brightness. Whether hardware problem or samsung OS problem, I got no idea.
Double arrow icon = data plan on. Go to settings, network and wireless, mobile network, untick packet data. Or just hold down power button and turn off data(2nd option). There may be times where that double arrow indicate wrongly, that is when you connected to data network, then it went out of service range, the icon will still be there but no data transfer can occur. Get it? For the sake of your credit, turn off your packet data now before you vomit to death.

Good information bro ! Already turned it off, dammit ! Means im running my data whole day today, DAFUQ !
Anyway, checked my bill. Not much charges so far... @.@
Oh ya, just now i went to task manager and clear RAM. After clear RAM there's a small window box saying how many Apps are cleared. i noticed that after i cleared RAM, when i press return to home screen. It lags, for about 1 or 2 sec delay. Why issit happened and someone facing this ?