Two: I've done that as well, when the traders still hadn't left for ages after I got the "the traders will be leaving soon" message I got desperate and tried it. Now that I know more I think they were just lagging up my game so bad that it seemed like forever. Does anyone know if it has the same effect the next year as siezing their goods does? That is, do they still bring less stuff or does it not count since it was done after trading was over?
On three, I wasn't sure from your description whether the stockpile was accepting other things or not, but if it isn't make sure that you have the quality and material settings right. Also, this is kind of obvious but it confused me for a while, if you set the options by turning "type" off and then turning large gems back on the pile won't accept anything. You have to leave type on and deselect everything inside the category individually. Barring any of those, is everyone too busy? Grabbing stuff from the depot seems to be low priority at times.
On four, I've had that happen as well. It was actually pretty funny, as civillians kept running up and putting what he had taken out but not equipped back in the bin. I could imagine these obsessive compulsive dwarves running up to the poor guy, ripping the armor he had just grabbed out of his hand and throwing it back in the stockpile: "NO! It goes HERE!" He managed to equip a helm and a couple other pieces, so I finally just told him to forget it and go spar.