The nails or screw are sunk because you are going to sand the surface after the mud is applied. You don't want screw or nail heads to be near as they rip sanding paper.
Sheet rock mud.
Question is unclear. A double sink can use one drain, you would anchor the base to the wall the same as a single sink and the vanity is on top or in front of the drywall. If you are replacing a single sink with a double, the supply and drain could be in the vanity, you would not have to open up the wall.
they are steel, steel sinks
with duct tape
slowly
The general rule is that an object will sink if it has a greater density than the liquid in which it is placed.
Because they are more dense than water.
A PVC ball valve, and add a threaded coupler to attach the supply line from sink.
Vents are run through the roof, shower and tub drains are installed, sink drains are stubbed out of walls and/or floors, washing machine box is installed, cold and hot water lines are stubbed out of walls and/or floors, sill-cocks are installed on exterior of house, and nail plates are placed over pipes going through studs so they will not be penetrated by nails or screws. Basically anything pertaining to plumbing that is inside a wall or cannot be done after drywall, tile, or flooring is installed.
A nail punch. They are flat on the point with a little dimple in the end. Different sizes for different nails.
You don't as it defeats the purpose of the AIR GAP Read your plumbing code
It may be the wastes that the plugs attach to which are leaking. Also look around the perimeter of the sink is it well siliconed and no holes? It can seep through there.