EK start to BV swap witch/Shadow help

I normal follow a PoB / Guide to the end game but i need someone to help me on this one plz.
I don’t know how to work with PoB very well and i decided that I wanted to start EK but ultimetly swap to crit BV or Elemental explosion BV(my favourite skill).
I decided this because both use a wand and scale Spell damage.

I do not know what skills to use to support EK during campaign and when to swap.

My other concern is to i start witch or Shadow.
Wich has spell/mana nodes;
Shadow will has easy acces to crit late game.

I also work for most of the day.

What should I focus on (guear wise) during campaign ?

Plz help and sry for the jumbled mess of ideas written without order.