I would think there should be a big difference between a slow ""depressurization"" and and a more sudden rapid and radical ""decompression"".
Perhaps, but this distinction is predicated upon your assumption that the definition of "depressurization" includes slowness, and that the definition of "decompression" includes radical speed. I don't accept that these are inherent in standard usage.
Case in point: when divers decompress (and they do use that term) they virtually NEVER do it quickly - that's the whole point.
One question I have is what would happen if someone replaced the O2 supply on the airplane with C02? Are there sensors monitoring C02 levels on the plane?
Kind of a weird question... What's your context for this? Why would it make sense specifically to have CO2 sensors? You could ask the same question about replacing the O2 supply with Nitrogen (another abundant diatomic molecule) or Argon (a noble gas), or any other available gas, but unless you're the Joker in a Batman movie, it doesn't make much sense.
Also, note that the passenger O2 supply in many passenger aircraft is not simply pressurized bottles of Oxygen gas, but rather chemical oxygen generators that initiate a highly exothermic (heat-producing) reaction that evolves oxygen. Due to the nature of these items, they don't look much like pressurized gas bottles, and so would be difficult to surreptitiously replace with a different gas.
If one were to be a pedantic word smith, one might think that a "compressed" cabin would be one where the pressure outside the vessel exceeded the pressure inside, for example if you trimmed the wings down to a stub, put the engine in the tail and converted your airplane into a submarine, which leads to...
Ha ha... pedantic word smith that I am, I might suggest an alternate interpretation, which is that it's not necessarily the container
which is compressed from the outside, as in a submarine, but the air within the container, which is (in a pressurized cabin) still arguably compressed, relative to the air outside.