Or it could be that the original mandate was to "fill the earth", not overfill it. If God gave us the ability to populate the planet then he can call a halt to procreation when that has been accomplished.
Just suddenly make everybody infertile, huh? Well, I'm not arguing that He couldn't do that. I'm just pointing out that the Bible doesn't say anything about Him ever intending to do so. And unless He did, you'd have overpopulation the likes of which we can't even imagine. The bottom line is that, as long as you're going to insist that the Bible contains the sum total of God's word, you've got a problem, because you can't add anything to it (like your assumptions that He had the overpopulation problem under control).
Who might conveniently forget that the Bible does not teach that we exist before conception?
I have no idea. While the Bible does not explicit teach that, it does suggest (in several verses) that our spirits existed before our bodies were conceived and that we lived in God's presence. Of course, to someone who doesn't insist that there could not conceivably be truths outside of what the Bible teaches, it's a total non-issue.
We come from the dust and return there...that is what God told Adam.
I keep forgetting that you guys are literalists. I'm assuming you're trying to say that both the spirit and the body are made from dust. Yeah, I don't buy into that, at least not with respect to the spirit.