In summary, it would have been easy to design the shuttle with crew escape
capability covering the vast majority of ascent/entry

I didn't think you could answer my question.

I didn't ask about ascent. I specifically asked about the stage at where
Columbia was lost.

The fact remains, that if the thermal protection system fails (tiles, heat
shield, whatever you want to call it) during reentry, the spacecraft is toast.
Nothing can save it, or the crew.
Bill Harris

Sci-Fi Quote of the month:
"We will never forgive and we will never forget." - Stilgar, "Dune"