1) You can build a stockpile for only 1 type of stone. Place your stockpile, look at it (q), go into settings (s), scroll down to stone, push (b) to block all catagories of stone, then hit right arrow a few times and look for limestone or whatever, then push (enter) on it. All sorts of stone are dissalowed except the one you want. You can even make it automatically fill itself by ordering the stockpile to take stuff (only what you have selected) from another stockpile.
2) If after 3 years you havent seen a single thief or ambush you probly generated a world where the goblins or kobalds died out, since I assume you have enough wealth generated to trigger something.
3) Automatic invader proof fortress (mostly) happens if you use: bridges, traps, moats (filled or unfilled), building destroyer proof floor hatches, hallways with ballistas at one end, drowning traps, any tamed combat animal stronger then a dog or marksdorfs behind fortifications. The enemy AI can't deal with those things at all so its basically an unintentional exploit. Use of any of those things for combat purposes basically == sandbox mode, assuming your defenses are correctly designed.
4) Easy mode is activated by: massed crossbows, selling goblin clothes, selling anything made from stone, using magma furnaces or magma in general, using water for combat, 1 tile large meeting zones, farming indoors, abusing the free embark barrels thing, farming tower caps if your map already has trees, cooking food out of alcohol, failing to start a war with the elves and playing on any map with less then high savagery.
5) easy mode is deactivated by relentless assault, or the orc mod, or things like that.