If you are wondering why some intakes have coolant flowing in them, its because they have crossover ports, head to head, or single heads, they are cooling ports, also these ports cool the engine block around the cylinders. If you are asking the other question, that means why does the engine lose coolant thru the intake, that's because a gasket is blown.
thermostat, or buildup of deposits in intake and thermostat housing
No there is not a water jacket in an intake manifold they are around the cylinder walls in the engine block.
replace the water heater inlet pipe on the intake manifold on a 3800
A bad intake manifold or manifold gasket can cause water to leak into the oil. Some newer cars have plastic manifolds and they will crack from age.
Not unless you have a water cooled intake manifold, which is very rare on a car.
The heated coolant comes out of the engine from the connection in the intake manifold and returns thru the connection into the water pump.
diferential pressure sensor intake manifold plausibility
It is a small 'L' shaped hose that goes from the side if the water manifold to the bottom of the carburetor intake manifold. I believe it is part of the water supply to the carburetor manifold that heats it.
Turbo charger
A turbo charger
intake manifold gasket if it is coolant in the oil
It does not use a MAF. It has a MAP sensor (manifold absolute pressure) on the front of the intake manifold.