For 3: Minecart Logic cells can create one-way hallways.
http://dwarffortresswiki.org/index.php/DF2012:Minecart_logicHere's a simple example: two hallways, both leading to a stockpile from which a workshop pulls. You want the dwarf in the workshop to enter the stockpile room through the north hallway, then exit through the south hallway.
########## [B = Raising Bridge]
WWWB ^=== [^ = Pressure Plate]
WWW####=== [F = Floodgate ]
WWW^ F=== [W = Workshop ]
########## [# = Wall ]Now build a Newton's Cradle memory cell from the link above.
Link the north pressure plate to the light green gear in the memory cell. Link the south pressure plate to the dark green gear in the memory cell. Link the memory cell's pressure plate to both the floodgate and the bridge.
Once this is done, the following will happen:
1. Dwarf paths from workshop to stockpile through north hallway since floodgate blocks the south.
2. Dwarf walks over pressure plate after the bridge, supplying power to the light green gear.
3. Gear activates roller and first minecart moves onto pressure plate in memory cell, activating it and bumping the second minecart onto the inactive roller.
4. Memory cell pressure plate raises bridge to block north hallway and opens floodgate to unblock south hallway.
5. Dwarf paths from stockpile to workshop through south hallway since bridge is blocking the north.
6. Dwarf walks over pressure plate after the floodgate, supplying power to the dark green gear.
7. Gear activates roller and second minecart moves up, bumping the first minecart off the pressure plate.
8. Memory cell pressure plate deactivates, lowering bridge, closing floodgate and resetting the whole system.
This method has a few flaws. First, anything following the dwarf through the north hallway has a chance to be crushed by the bridge raising. Second, the floodgate can be stuck open if a dwarf drops a sock on that tile. But it does create a one-way hallway into a room, and a one-way hallway out.
Edit: After reflecting, the danger of being crushed by the bridge can be avoided if you replace it with either a retracting bridge over a pit or use a hatch cover instead. The pit will be just as impassible as a raised bridge, but anything that happens to be on the tile would instead just drop into the pit below, from which you can make an exit elsewhere. If you want to avoid another dwarf entering the stockpile room from the south after the first dwarf has activated the system, make the hallway longer and increase the number of tiles between the pressure plates and their blocking structures. That way, if someone does try to enter from the south, walking over the pressure plate in front of the floodgate will close it before they can get through, forcing them to go back and enter the north instead. Likewise, if the dwarf inside tries to leave through the north after the second dwarf toggled the system, he'll toggle it again and be forced to go south.