Open up another outlet down the line and see how many ground wires are in them. you should have 3 if its in a metal box. 2 if its plastic.

Is this used over the plastic sheet or is a plastic sheet necessary on the studs with the cement board? 

We either had to enshroud the house in plastic while working or don't do the job.

The plastic fittings seem cheaper but can crack. 

The runs to the fixtures are also a combination of copper and some kind of plastic (not sure about this one). 

There was a plastic sheet (black garbage bag) taped up on the ceiling, when removed, revealed a water spots and some mold forming maybe about a 2ftx2ft area.

Spray the dark spot with peroxide, cover with a soaked peroxide rag, cover the area with plastic wrap over night.

I have a degree in plastics engineering, a strong knowledge of carpentry and home improvements and an entrepreneurial spirit that has been growing since I started my own lawn service at age 12.Currently, I am taking the courses necessary to sit for the PA Real Estate license exam.  

Buckets of moisture are migrating up out of the soil into your investment(lay a 2' x 2' piece of clear plastic on your lawn in the middle of summer and watch how quickly water droplets form on the underside). 

If they are metal and sometimes plastic, you might check into resurfacing it.