If it's ambushers you're worried about, chain a couple of kittens or any animal really somewhere outside the fortress, preferably far outside the range of where your dwarves travel, this will act as an early alarm system for ambushes.
If it's sieges, and to your siege question, when it's a siege, multiple groups show up, that particular group with the swordsdwarf just happened to be led by him, but he had a past injury, so they won't go anywhere because their captain can't.
But more groups showing up isn't because you don't kill them, they just show up as part of the siege, depending on your fortress wealth, past sieges, etc, you could get anywhere from 20-80 goblins if I remember correctly, that lower number may be off though.
As for outdoor defense, there really isn't too many tips I can give you, siege operators not firing could be because you gave a "no civilians go outside" order, which means, unless your siege engines are below ground, your dwarves won't touch them.
Also, siege engines can't fire between z-levels, so the level you put them on is hopefully the level that has invaders.
As for effective range, I do believe they will continue going until they hit the edge of the map.
If you want a cheap, basically cheating defense, build a bunch of bridges right outside your entrance, make them all raise to the side, and then just slam them down on any ambushes/sieges.
If you want a better defense, more military will help you there, if you have over 70 dwarves you should at least have 15 or so military, IMO, some people would shoot for even more.
And, last but not least, build a hallway as your entrance, line it with traps, this is a cheap but effective way to repel sieges and ambushes.
Also, try only letting your dwarves outside to gather stuff directly after a siege, you only get one siege/ambush group per season, I think, but if it's taking your dwarves more than a season to gather stuff, well, that might be a problem in and of itself.