Ah. So the connection also counts diagonally across Z-levels as well?
Yeah, sort of, though I kinda forgot that staircases support, too. Which is relevant because of how I'm interpreting your diagram below:
but actually, the floor below it is like this:
<<<<<<<<<<
<========<
<========<
<========<
<========<
<========<
<========<
<<<<<<<<<<
because I see those <s as up stairs, which work just like floors for support. So get rid of them and it should collapse.
And the center is all channeled out because the aquifer is one more down.
By that you mean that there's a z-level where there's just an open pool in the aquifer, right? That would be the logical thing, and is the correct way to go about this method of aquifer piercing.
Also, the fact that you don't know how to cause a cavein, or how you could've saved that dwarf given your apparent and probably resources, suggests to me that you probably shouldn't be trying to pierce an aquifer just yet. It's not an easy thing to do, and can be terribly disastrous if you make a mistake.
(You could've built a wooden bridge or floor over to save the dwarf, since you obviously have at least 1 tree and probably have an axe. The bridge wouldn't even have screwed up your collapse plan, since they don't provide support. Just FYI
)