How about the msnbc story that I linked and quoted at post #28? http://www.msnbc.msn.com/id/35428010/ns/travel-news/#.TvJxrlZ4kyc TSA didn't dispute the family's story that the boy had been forced to remove his braces.
It's true that people presenting at the checkpoint in a wheelchair have different levels of ability, but here's a nugget of wisdom @:-) I learned on the Disability Forum. People who use a wheelchair in daily life due to inability to walk even short distances have their own wheelchairs. Also, in many cases, they either roll the chair themselves or have motorized chairs. After all, they have to do this everyday outside the airport. For the most part, the people you describe above are the ones using the airport-supplied wheelchairs and being pushed by an airport/airline employee. I was one of those myself for two trips - one to the US and one via Asia to Europe - prior to my hip surgery. If someone shows up at the checkpoint in their own wheelchair without an airport 'pusher', it's a pretty good bet that they can't walk well enough to get through the checkpoint unaided.
I agree that they should ask the passenger, but asking "can you stand?" is setting that as the default. It implies that the screener would prefer them to walk through rather than screening them in the chair (I'm sure they would - it's less work for the screener!) It puts the passenger in the position of insisting on being treated differently. And in the above, as soon as the passenger says "I'll try" instead of "oh, yes, of course I can walk that far", the screener should say "If you're not sure, I can screen you in the chair." Your scenario focuses on convenience for the screener instead of the safety and comfort of the passenger. (I'm sure that accurately describes the TSA, but it shouldn't.)

The same could be achieved by saying "I can screen you in the wheelchair if that's easier for you" and let the passenger say either "yes, thank you" or "it's okay, I can walk through." It demonstrates that the screener is willing to be "respectful and sensitive" as their HQ keeps claiming. Start from the assumption that the passenger can't walk and allow some to say "I can," rather than starting from the assumption that they can walk and forcing some to say "I can't."

And here's the difference, in general, between my (admittedly limited) experiences as a wheelchair passenger with TSA vs Australia (SYD), Asia (BKK, SIN) and Europe (FRA, GVA, LHR). I could stand and walk through the WTMD, and I figured that was going to be easiest. In the US (LAX x2, DEN), before I could even say "it's okay, I'll walk through" I was asked to get out of the chair (possibly because I was in an airport chair). In Australia, Asia and Europe, without exception, I was asked whether it would be more convenient/comfortable for me if they screened me in the chair (even though I was in an airport chair and pushed by an airport employee). In FRA they insisted that I stay in the chair while I got a rather cursory patdown and wanding.

Most of the problem, apart from poor training, is the risk avoidance attitude of TSA compared to risk assessment and common sense demonstrated elsewhere. I'm not saying that people in wheelchairs shouldn't be screened at all. But what happened to my friend's father - an elderly Australian man in his own wheelchair - and Lenore Zimmerman, Ryan Thomas, Thomas Sawyer and all the rest suggests that too many TSA screeners see such people and think "they must be hiding something - better give them the works!" Respectful and sensitive?