The way I see it, is that d_init is the list of options in the F10 menu that DF doesn't have. Instead of having a menu in the game, it has a .txt. I wonder how many people would use the population cap if it were adjustable while running... Plus, it's really not cheating. Counties do this A LOT, just tell the motherland "nah, we've got enough people right now, you can keep them." Since the population cap is 200 by default, you pretty much HAVE to change it, or else you suffer FPS death. The same applies to the baby cap. See: China, "We've got enough people, I'm issuing a decree to stop breeding so we won't starve ourselves."
Also, try generating less wealth. High created wealth means more migrants, so if you work with wood instead of stone, and stone instead of metal, or simply don't produce things, you can do pretty well. I can run an "under the radar" fort for ~2 years without seeing a babysnatcher, and only get ~40 pop, just by wasting time chopping some wood, building some wooden walls, getting a farm set up, walled off, and farmers trained, and sometimes even plumbing running using wooden pumps. When you start producing nice stone tables and metal armor, then you attract attention from migrants and goblins, but if you're patient and give yourself time, you can build a thriving agriculture and THEN a metal industry. Just remember that when you dig out stone, that stone counts towards your wealth, so digging out gems and iron ore can get pricey.