Two problems with your analysis. First, there simply isn't enough fuel left in Columbia to reach the ISS (the burn required would take more fuel than Columbia can stuff on-board). Second, until no earlier than Columbia's landfall over California on its descent, every indication, including the evidence from a similar event 2 launches prior, was that re-entry would not be affected.
You seem to know a fair bit about this, so I'll ask you - they said they didn't have the robotic arm, did they have the necessary equipment to perform a spacewalk in the first place?
Secondly, if they had performed one and discovered significant damage it has already been pointed out that there wasn't much they could do about it given the fact that they couldn't make it to the ISS and the difficulties of launching another shuttle in time to make a difference, as you've outlined.
Nevertheless, it would have been nice to know what they were dealing with so that they would have a better idea of what would/could happen during reentry.
The NASA guy made a point of how they had investigated on the ground (with people experience with the tiles) and thought it would be okay. Thanks for your insights.