Actually, having performed a few cave-ins when making my "water tank" for a particularly high-elevation subterranean river, I can say I've seen them collapse non-instantly.
They ARE, however, very fast. I think they work by dropping one floor per step/tick.
Cave-ins also produce dust clouds and fling objects. This is what killed some of my dwarves, in fact, because the channeling they did cause a dust cloud that flung them off the ledge they were standing on, plummeting down 22 z-levels to their doom. Objects like stones will actually be flung somewhat upwards, as well, if they aren't squished and immediately destroyed by falling floors from above. This meant that after my dwarf fell to her doom, her arms and legs and head bounced about the chamber before coming to rest, then extra stones fell down, burying the body.
Anyway, you're mistaken if you think you need to put water-based timing mechanisms anywhere near the things they control. Mechanisms work by plain-out telepathy. They require no connection to the large, heavy objects they move, and no power, either.
I would suggest making your tower have small segments that collapse one at a time within the tower itself, so that individual floor tiles may fall away, one at a time at first, then a few at a time, then whole segments of the tower, for that nice, cinematic effect.
The easiest way to do this is to set up a pump (or even just a floodgate or door) near a large enough source of water, a long channel, and line that channel with pressure plates. Each pressure plate along the line would trigger the support to fall away as the water went down the channel. Provided the water is not pressurized, this shouldn't even be terribly quick, giving the adventurer time to make an Indiana Jones-type escape. (Set stones around collapsing parts, so that they come falling down as your adventurer tries to escape with his life.)