Who said anything about going to Darkness spec? Just get a shield generator, switch your charge, and dps as normal. Oh and do all the other little things you should do, interrupt, stims, buffs, and the such.

No, not if you learn to interrupt and if you are having trouble with a boss, put on a shield generator and turn on Dark Charge for the duration of that fight.

Or get a shield generator and turn on dark charge for fights in which you want survivability.

Not to be an ***, but you are playing the Deception Assassin wrong if you think most of the builds dps comes from Maul. Maul should only be used if the duplicity proc is up and occassionaly if coming out of stealth and you know that a maul and assassinate will take the target down. Most of the DPS from deception comes from VS, shock, and discharge.

There is a respec trainer in the same area as the guild register trainer at the Fleet (they are in the furthest right area where the class trainers are).

He is specced madness so he does not have VS.

Yes, Lighting Charge can Crit (see Darkness skill Charge Mastery which for Lighting Charge increases the crit chance by 3% for each point put into it).