I am a latecomer to this thread, but I have read the available posts and will attempt to answer your questions, bearing in mind what I think you are trying to prove/disprove.
Originally Posted by SUPERMNNN
2. The transponder signals were more likely accidental, not related to the airplane landing into an airport. It was most likely not pilot related, i.e. indicating that if our pilot was in control or not.
It is quite possible for the transponder to be switched on again during a struggle in the cockpit. I am not familiar with the 757 cockpit, but the the controller for the transponder could be on the centre console where it could be bumped.
It is also possible that a pilot who has regained control would switch it on, but that would probably not be his first action. It is much more important to first fly the aircraft and then later establish communications with the outside world - again probably by radio first.
Originally Posted by SUPERMNNN
3. No professional pilot and air traffic controller would plan ahead, thinking of landing a 757 into this small airport.
That is very difficult to answer, but again, unless the pilot is rated on type, he would be better off to first get away from the ground, learn to control the aircraft, make contact with people on the ground and then attempt to land.
Originally Posted by SUPERMNNN
Considering that at 10:00, the plane was at an altitude of 7500, the plane crashed (to the altitude of 0) at 10:03:11, then if the two signals were between 10:00 and 10:03, 6400 ft and 5800 ft,most likely the tansponder gave the correct altitude reading, obviously it did give the correct flight number, therefore we can assume that the transponder was functioning but most likely switched on by none human activities, "unless specifically asked to do so by ATC, or if you want to squawk the emergency code."
Those altitudes are pretty low. I don't know how far the ATC centre receiving the signals was from the aircraft, but it is conceivable that the transponder was on during the times mentioned and those were the only two reads that the receiving equipment got. Transponder signals are line of sight because of the high frequencies used and the signal is easily lost if the aircraft is low and far away.
Originally Posted by SUPERMNNN
Does the ATC normally ask you for transponder signals before landing, especially for emergency landing?
The transponder would normally always be on in flight. ATC will not ask you to squawk a different code before landing or in an emergency, because their equipment will already tell them who you are. If you are in contact with ATC and your transponder is off, they will probably give you a code and ask you to switch it on.
Originally Posted by SUPERMNNN
Is it possible for our pilot to use the tansponder to inform the ground that "we are in control"? It seems to me, from Tom Brokaw's interview, that it was a pretty loud signal to the controller. It might not be a big deal for a regular flight or emergency.
It is possible, but unlikely. Using the radio would be far more probable. Both an ATC and a commercial pilot would know the guard frequency which they could use if they had no contact on other frequencies.