Thanks for your informative post. In addition, there is an issue that I was unaware of until I read it in the original article here: "...Challenger's external fuel tank disintegrated, throwing the orbiter into the local airflow at forces way above design tolerances. It ripped apart, claiming the lives of seven astronauts."
Implying that even if Challenger could have been separated cleanly and undamaged from the exploding tank, it would not have survived. A stronger, and therefore heavier or smaller orbiter, does not seem to be feasible, given the extent to which the shuttle design was already stretching the envelope.
That's why we are back to capsules, I guess.