"For the feeder chute, a retracting bridge at the top will not work because it will act as a "bridge-a-pult"."
How? When retracting bridges retract, they vanish - it's the drawbridges that form walls.
For the fortress purge system, draining water into magma doesn't actually work because it'll just create an obsidian plug - if you want to get rid of the water, you'll need to drain it off the edge of the map.
Not if I drain it into the semi-molten rock, it won't. Speaking of which, I'll try to integrate an obsidian creator into the drainage system.
For the feeder chute I would recommend forgoing the upper level bridge and just setting up a minecart stop quantum stockpile system to auto dump down the chute. Keep the lower bridge/hatch as that can still be useful to limit the incidents of dwarves getting hit on the head with falling supplies. The advantage of this approach is you never have to send a bunch of dwarves up to dump the supplies down the hole the haulers that bring the wood to the stockpile auto handle everything. I don't understand minecarts terribly well, which is why I don't use them unless I'm certain that I know how they will work.
Just make sure that there is a wood stockpile on the level below the chute so dwarves don't try to haul the wood back up to the top.As the whole point of the chute is to bring in cargo from upper levels to stockpiles below, I don't think that I''l have a problem.
As to the fortress purge... you pansy, and you call yourself a dwarf. Real dwarfs would build that with magma not water. ...Magma which would incinerate all of the buildings in the room.
Generally a system of bulkheads so you can seal off problem sections of your fortress are more than sufficient. Something nasty got into your dining room? Seal it off and dig out another. You can always send your military in to deal with it or find a way to channel magma into the room at a later time without dealing with fort wide flooding devices.I'm planning a bulkhead system. As to 'fort-wide flooding', my poorly drawn diagram shows different ceiling grates for different rooms, though that was hard to convey in 2D. Nevertheless, every room, corridor, and stairwell should have it's own drain and flood openings, and are sealed off by doors and drawbridges. I am not looking forward to the lever network for those, let me tell you.