1. "A violent jarring; a shock", basically includes things like high explosive grenades etc, but also includes sonic weaponry and also things that if you were hit by irl you would have a really bad concussion like wrenches and crowbars.
2. Not quite, has different resistances but it tends to cause stun.
3. Default hotkey for search is Q.
4. Really depends on what you're up against, I recommend looking at enemy armor values and such and making informed judgments from that. HE = explosion and BS = buckshot meaning like a shotgun spread.
5. Mainly just experience in combat, basically anything will improve strength and time units, but there are also special transformations that can increase them as well.
EDIT: Note that BS ammo is basically multiple smaller pellets being fired at once as opposed to a large shell, so if you're dealing with something more heavily armored, typically BS ammo is not the way to go.