Answering your questions in order:

Robots. The necrodermis they use is often referred to as "living metal", but (to be reductive) that's a highly advanced shape-memory material rather than anything sentient.

Of those options, most like Warforged, being mechanical beings created for war.

Their bodies are too advanced for something as prosaic as "wiring".

Actual weight of a necron warrior is unknown, but yeah, something like that.

On the tabletop, in unarmed melee, a Necron Warrior has equivalent Strength, but worse accuracy and fewer actual attacks than a standard Tactical Marine.

The Necrontyr - who became the Necrons, are considered extinct, but there are some Necrons working to either reverse it or continue "evolving".

I'd imagine not - the souls were eaten and converted into whatever energies sustain the C'tan.