Visitite is indeed a known metal source, and so far the forum hasn't found any effects of slaughtering them (but that might change in any release, of course). Since there are parameters for how many visitors you accept I assume you can increase the flow (at the expense of FPS and "undesirable" visitors), provided your world supports it.
Given that military personnel now (0.43.05) carry full sets of armor, the amounts should increase considerably.
Personally I don't use that source intentionally (certain undesirables need to be eliminated, and I can't do anything about stupid military types that decide a goblin siege is a good time to arrive/leave).
Possibly, but consider this:
you could send Recruits with no weapon skills up against unarmed/armored dancers/poets/etc, and let them beat them to death with their fists or a blunt weapon. When they start OHKOing them, it's time to switch to a different weapon, to get the base skills up (Fighting, Dodging, Strike, etc). If they're properly armored, it's highly unlikely that they will be injured.
Then, when you think they're ready, send them out against the armed but unarmored mercenaries.
Don't think of it as a metal delivery system.
Think of it as a made-to-order danger room that can be begun with minimal resources and difficulty, the ability to reset it every few weeks, and you can clear the difficulty at any time with veteran troops--that happens to occasionally drop weapons-grade metal. I wouldn't necessarily want to try it early-game without armor, because our old friend Murphy could visit at any time.....