When you're cloaked and behind a target, you'll get the "backstab" instead of the standard autoattack.
Backstab deals double damage, so if you're keeping yourself stealthed you're maximising your damage by using backstab and reenter stealth.
The 3.) is possible a matter of style. Some people tend to use Daggerstorm while others use basilisk venom.
I've been using it the last BWE.
You spawn it (with a circle) somewhere on the ground around you, though you (or someone else) are able to use it for 60 seconds in whole.
After that it just vanishes.
The first few minutes when spawning for the first time in the asura map and later on when 40 vs. 40 people clashed in WvWvW.. this was somehow annoying because it worked fine for me in BWE2..