1. Chest and diaphragm muscles can only expand the thoracic cavity if the internal and external pressures are equal. If the external pressure is higher than the internal pressure, the chest will actually collapse inwards to some extent, until the pressures balance. SCUBA regulators are designed to deliver air at ambient pressure, allowing divers to inhale easily both at the surface and while at depth.
2. There are several potential dangers. During the descent (increasing external pressure), it is important to regularly equalise the body air spaces (lungs, sinuses and middle ear) with the surrounding water pressure. Lungs are equalised simply by breathing; but deliberate action may be necessary to equalise the other spaces--failure to do so can cause pain, bruising or eardrum rupture.
A more insidious risk is that nitrogen in the breathing mixture becomes increasingly narcotic due to its increasing partial pressure (this effect starts becoming noticeable at approx. pN2 = 2.8 ata; when air is used as the breathing mixture, a depth of ca. 25 m, 80 ft). This can lead to poor judgement and decision-making, and hence in the event of an emergency may increase the likelihood of accident.
Similarly, oxygen becomes increasingly toxic above pO2 = 1.4-1.6 ata (on air, at around 57-66 m, 185-215 ft), poisoning the central nervous system leading to convulsions, loss of the regulator mouthpiece, and drowning. These effects are predictable, and thus generally avoidable (by limiting depth, or modifying the percentage of gases in the breathing mixture).
During the ascent (reduction of external pressure), the most dangerous effect is the expansion of gases within body air spaces. If this expanding gas is not allowed to escape (e.g. because the diver is holding their breath as they make a panicked ascent), then it may tear through lung tissues, and even enter the bloodstream, blocking blood circulation and leading to breathing difficulties, pain, paralysis, or even death.
At the end of a deep and/or long dive, or series of repetitive dives, inert gas (usually nitrogen) which has dissolved into the body tissues must also be eliminated in a controlled fashion. This may entail slow ascents, possibly including mandatory stage-decompression (stops). Failing to adequately eliminate this gas prior to surfacing may result in bubbles of gas coming out of solution in the tissues/ bloodstream, again causing blockage and/or damage. These two conditions (lung over-expansion injuries and decompression sickness ['the bends']) are collectively called 'decompression illness' (DCI) and require prompt treatment in a recompression chamber to prevent/ minimise long-term damage.
3. The buoyancy compensator (BC) is a sealed flexible bladder with a combined inflation/ deflation valve, which a diver uses to adjust their total displacement volume (i.e. buoyancy) throughout the dive, allowing them to compensate for changes to their weight/ buoyancy caused by consumption of cylinder air (a full dive cylinder weighs approx. 3 kg more than an empty one) and compression/ expansion of their exposure suit during descent/ ascent. This is done by adding air to the BC to increase the amount of water displaced by the diver, and hence their buoyancy, and releasing air to reduce buoyancy.
Underwater, a diver aims to be neutrally buoyant (neither floating up nor sinking down, i.e. effectively weightless) at all times in order to minimise physical workload, and make controlled descents/ ascents (to minimise the risks described in (2)). At the surface, a diver aims to be positively buoyant (i.e. floating) in order to rest and relax before and after the dive.
Former full-time SCUBA instructor (PADI MSDT # 609394)