[Dwarf Fortress] The "fun" starts at 2:23!

Edit: Solved the issue; solution at bottom.

Look, internet; I put the floodgate and switch _because_ this could have happened. I would have kept the dwarf stationed by the lever and just let out enough to test and not flood the place. I just wanted to record my triumphant "it worked!" video. Got something even better. Now, I didn't think THAT would happen so I got some thinking to do.

So, I did the rest of the attempts "live" with a dwarf on standby with the lever. Good thing, too, because I tried a few potential solutions to the problem that all failed. After observing the timing of the flooding, a lot of lever flipping, and patience I realized that despite seeing rocks in the trench flow off the map that the flow issue had to be there and THERE was the issue. I was one block off setting up the off-map grating. Now, I have _not_ tested this yet but confidence is high!