Do I just take power from one leg and ground? If so, how do I ground the outlet?

I was changing an outlet in my house yesterday and found out that the romex coming into the box had no clamp. Is it possible to add a clamp from the inside of the box, without removing the wiring? It's an older box with no place for a built in clamp. I'd need to add a round one.

How do I properly anchor this wire in this type of outlet box? I am thinking that because this outlet box is not going to be behind drywall the NM cable must be anchored. That's what my favorite book says is this true or do they just mean J-boxes? I have round plastic push in kind anochors for...

Using outlet box with power and 3 neutrals(tied together/insulated) I want to use for adding a light fixture. Separated wires and found voltage on one of the neutral wires (measured ground wire to N). Any condition where power on the neutral lead is OK?

I am moving a electric heater thermostat and need to extend the exhisting wiring. The exhisting wire is 12/2 can I extend with 10/2 wire? Is this safe since I am going from a smaller gu wire to a larger grade wire