[eno_]

"why didn't they do a walk to check the condition of the wing?"

There are no handholds outside the cargo bay. The robot arm might have been able to "look" back there, but it was not on this mission. Their options for diagnosing tile issues were ground-based telescopes or a flyby of the ISS, and they may not have had the maneuvering fuel to get to the same orbit as the ISS.

[Republic of Texas]

Columbia was too heavy to get to the ISS, too heavy to dock with it, and didn't have the fuel. The ISS doesn't have engines to get to them. It simply wasn't possible to "park" it and wait for Bruce Willis to come to the rescue.