Rules clarification about overthrow, tagging up, and award of bases?

Correction. On an overthrow, the batter gets one extra base. Unless it's ruled he would have gotten to the next base safely. For instance, if the runner hits a ball to third and the third baseman's throw to first is high and goes out of play, the runner gets second because the ump would rule there's a good chance he would make it to first if the ball had stayed in play. In your case, the runner would HAVE to tag first and he would get second. So even if he stayed on first and the outfielder for some reason threw to first and it went out, the runner would get second.