1) This assumes that the no-fly and selectee lists actually provide added value to the screening process. I'm not convinced that they do.

If someone is dangerous enough that they shouldn't be flying, then charge them with a crime and lock them up. Otherwise, it would seem more efficient to focus that effort on making sure they're not carrying weapons aboard the aircraft.

2) Let's stipulate, for the sake of argument, that checking against the no-fly/selectee lists is going to happen anyways. If, as you posit, the computers at the TDC stations would have access to those lists, there's no need to validate the boarding pass. The passenger's ID should be sufficient to verify the person's identity and their (non-)existence on the various lists. The airline will validate the boarding pass at the gate; they're the only party with a vested interest in knowing that the boarding pass is legitimate.