So, there are no humans on this planet, and you can’t bring anything with you (or presumably back) except a person with no clothes or tools? Aside from, perhaps it’s use academically, I’m not seeing any benefit to doing much with this place. I’m not sure how the humans you propose to send through would fare…I suppose they could build rudimentary weapons out of stone, but even a survivalist is going to have major issues with zero tools in a totally untouched world. It would be a major struggle just to survive, let alone try and do any advanced technology. Hell, even something like gathering copper and tin to make bronze would be an epic task.
So, the answer to the 2nd quest is probably ‘never’…or at least not for a very, very long time. You’d need to send colonists through, and those colonists would basically have to focus mainly on survival. That means hunting and gathering, since you won’t let them even bring seeds through. But modern humans aren’t really adapted to hunting and gathering, and even our remote ancestors who actually did this didn’t start off with literally nothing. You have to go back to way before humans before you had no tools or tech at all. Sure, some survivalist types could nap flint or make a fire and, perhaps eventually build enough to survive well enough and have enough time to expand that.
You don’t need to worry about wrecking the environment, however. I’m not sure what your expectations are, but if you are thinking you could even build a small steam engine or something along those lines you are way too ambitious. You’ll be luck if the folks you send through are still alive this time next year.