That is a very interesting question. My hypothesis is that the pressure would drop to just a bit more than 1 atmosphere as you moved the gauge just under the "roof".
Think of it this way. Fix your pressure gauge in position just under the roof somewhere away from the pipe. Take a reading. Now double the cross section of the pipe, so it's only half as high. Did your reading change?
When you say "double the cross section, so it's only half as high," you have in mind that the increased cross section causes the same amount of water to spread out a bit horizontally, right? In that case, yes, the pressure reading decreases.
Now excavate just enough of the roof for the entire column of water in the pipe to collapse into the pit. The gauge is now under a couple of cm/inches of water. Did the reading change?
If you dig out a little piece of the ceiling, the water in the pipe will go down a bit. So yes, again, the gauge will read a smaller quantity of pressure. If you continue to dig out the ceiling, bit by bit, the gauge will continue to decrease. Eventually, the volume of the ceiling will exceed the original volume of water in the pipe. At that point, further excavation will have no effect. So there is a diminishing return here, that asymptotically approaches zero
I would expect the pressure reading to stay the same in all three scenarios. How could it be different? The all have the same weight of water above the gauge.
It can be different if the weight of water above the gauge is not the relevant quantity.
Why would the shape of the column matter?
It doesn't. That's the OP's question. What matters is the height delta between the gauge and the water surface.
Maybe an example of a flexible container will help illustrate -- we're all divers here, we've thought about the underwater balloon. Consider a flexible, sealed balloon with a fixed amount of air in it, say the size of a volleyball at the surface. That's about 8 inches in diameter. I think we would be in agreement that if you submerged that balloon, the pressure would increase, and it would cause the air inside the balloon to compress, so the balloon would shrink. Say you bring it down 10m / 33ft, the balloon should have shrunk to about half the size, about 4 inches in diameter, like a grapefruit.
If that point at 10m/33ft in your picture is where the thin pipe meets the large basin below, what happens if we move the balloon around:
1) If we move it to the right 10m/33ft , does it change size?
2) If we move it straight down another 10m/33ft, does it change size?
3) If we first move it to the right, then move it down, does it change size?
It sounds like in your mind, the balloon would have to decrease in size as you move it to the right. Have I understood you correctly?
In my mind, 1) is no, 2) is yes, and 3) is no for the horizontal movement, and yes for the vertical movement