[AntEater]

I'll answer some I know

2. Since Trieste was in US navy service, you can say the maximum depth of a military sub is 10,500 meters.
The woods hole submersibles (Alvin etc) can go around 6000 meters or so.
However, maximum realistic depth for a combat submarine was the russian Komsomolets with around 1500 meters.
Normally today's SSNs operate above 300 meters.

3. In larger submarines, the air supply in the sub itself can sustain the crew for a day or so. Depends on the size of the submarine and the number of men inside.
Oxygen bottles were normally carried in WW2, not sure about today.

4. Since torpedoes do not have any air compartments, depth should not affect them at all. Maybe their components could be damaged by the pressurisation process, but as soon as they are in the ambient pressure, they should work normally.

5. Around 1000 meters (can be lower or higher), there is the deep sound channel, where sounds travel the slowest. In this area, sound travels the widest, sometimes around the globe.
Below that, sonar conditions are similar to the surface.
However, changing temperature in the water can distort sound conditions at any depth (geothermal vents, for example)

6. Below certain depth, submarines are noisier, because of the pressure on the hull.
Also, in a hypothetical deep diving scenario, submarines would avoid fast depth changes because the pressure change could cause damage, or at least noise.