#3 seems safe enough to answer. Sky Marshals are Federal Agents and specifically are part of the Department of Homeland security. As far as I can tell, they are pretty much the only worthwhile part of that agency.
exile3D, you’ve started 3 threads so far. One about breaking down pilot doors in flight, one about a conspiracy theory, and this one one about Sky Marshals. Are we going to see you on the news soon?
The short answer is - they aren’t going to tell you or me what the details of security arrangements are.
As for taking over planes, breaking down doors, fights with sky marshals… The 9/11 hijack is one of those “we can only do it once” trick like the old Daffy Duck cartoon. Up to 9/11 the standard behaviour was “cooperate, land, negotiate, eventually things are settled and we all come home safe.” Since then, there are plenty of tricks a pilot can do to make someone regret they were not buckled in, if things in the cabin get difficult. It’s tough to hold a hostage and a weapon, or chop a door open, when you’re bouncing off the ceiling several times a minute, and nowadays pilots are motivated to take evasive maneuvers.
AFAIK - the windshield and windows are shatterproof glass, so you’re not going create a massive decompression like we see in the movies with a bullet hole. Even less likely with a hole in the hull. I assume they use lower-velocity bullets anyway that will stop a body but aren’t going to go through several bodies and a few layers of metal.
Of course, once shots are fired, the aircraft will land at the nearest available airport and be surrounded by SWAT teams, so a minor air leak will not be a major problem. If necessary they can descend to cruise at 12,000 feet until they reach the airport, so that air pressure is not a problem.