I hope you have detailed shots of the interior, every square inch of wall, ceiling, and floor.

Find what works for you and chip away every day inching toward your goal.

When I moved in I power rodded the 2 inch pipe sections and flushed them with boiling water and green gobbler solution and i havent had any issue since i moved in about 18 months ago. 

I suggest there needs to be an open conversation with this "GC" to get real clear on who and what he is, and what your expectations are, well known or not, before going an inch further.

Heather, if you give them an inch they will take a mile.

It’s an old building and sits on at least 8 inches of a concrete slab (what I have seen visually during some landscape work.)The structure does not have plumbing, and I would want  plumbing roughed in for 1 bath, kitchen and washer.

Keep your head down and move forward no matter what .. even if it feels like it’s just one inch at a time .. always move forward and think on the end goal ,not today’s problems !

If it has the old 6 or 8 inch tiles those are asbestos.

To differentiate between pressure and flow, pressure would be measured in pounds per square inch and flow would be gallons per minute.

You must maintain a 1/4-inch-per-foot slope tie-in to the sewer.