Thanks for helping on this Watercolor. Here is the same shot but of the entire photo. Here are answers to your questions...
1. Inside the hole below the liner and core materieal, a bronze washer sits on the inside of the hull and a bronze nut sits on the washer. The nut is threaded on the thru-hull. Then the valve sites on top of all that.
2. The nut is quite large. A 1 1/8 wrench fits it. Although, the nut could not be reached with a wrench on the two thru-hulls in this location, since they are recessed below the liner and core.
3. Good question on the carriage bolts. I know they don't go to the outside of the hull. On the other hand, I can't tell you what they attach. I will investigate.
4. This location is under the Galley sink. This particular thu-hull intakes water from outside the boat. There is one for drain that can't be seen in this photo. In the photo below, you can see the electric and manual water pumps.