The hand Icon shows your "Arm Stamina". It will go down when you put your gun up and the lower it goes, the more unstable your aim will be. Arm stamina recharges when you use the 'C' Key to put the gun down. Also, if you get close to corners of buildings, rocks, trees or anything else that is solid you will automatically brace your gun on them, boosting your aim and significantly reducing the degradation of arm stamina.
Radiation zones are tricky. I usually take it slow, keeping my lights off most of the time and try to listen for Tentacles and enemy Raiders.
Night Combat in this game follows some of the rules of real life night combat. So you want to keep your Lights and Lasers OFF for most of the time, since it will give your position away. Only turn the lights on for a short time when you are engaged by an enemy, to try to blind him, or when moving through Terrain that you do not know. The enemies will usually have their lights on as they roam around, making it fairly easy to spot them.
Blue light will, like any light, will give away your position. The aktual advantage of those Lights is that they are better at shining through Fog, giving you a slightly better view range, when it is on.