3. For any metal item, it's best to melt it down into bars, but anything else just clogs up the place. An alternative is to trade the leather and silk items (they frequently have giant spider silk -item-s which is $600 a piece), but it will never increase your Exports; important if you want more nobles.
A neat alternative to chasming everything (if you want to keep the natives down there alive and well and happy) is to dig yourself a chamber and stockpile all the non-metal foreign items there. Then lead a shaft of lava down to that chamber and open a floodgate. There, problem solved! Just a wee bit of nonexistent residue and it's ready for the next load.
4. I had the humans decide to siege me after 1 or 2 of their trade caravans were slaughtered by goblins. There's a certain level where if their losses break that point, they will seige. I'm not sure if they stop, however...
5. Elves ambush, yes. I might have been hallucinating, but I think they come onto the map in a pack stealthed, until they start shooting. Then you get an announcement, where you panic and get your dwarves scurrying into your fortress. After that, it's no different from a siege, other than the fact that the Elves aren't at all interested in moving into the Fortress; they are quite content to sit at the mouth, ready to make a porcupine out of the first thing to poke its nose into firing range.
You can fry them with a magma flow, but you might run into the problem of an infinite flood, making the outside unsafe for ANYONE.