1. You always need your supply air to be at some absolute pressure above your ambient pressure. Since the pressure increases by an atmosphere for every 10 meters / 30 feet (approx.), at that depth you have no more supply pressure. At exactly that depth you could in principle still suck air through a wide tube, but see point 2 below.
2. There are straightforward formulae for the pressure LOSS for a fluid or gas with a given viscosity through a tube of a specified diameter and length. The details depend on whether you are in the laminar or turbulent flow regime (Reynolds number). So with a finite tube diameter and the viscosity of air being a known quantity, you can calculate the pressure loss for a given flow rate such as that involved with breathing. You must supply an overpressure to overcome this pressure loss; of course this is why our standard scuba regulators deliver a certain pressure (relative to ambient) from the first stage and then down regulate further with the second stage to a breathable ambient pressure at depth.
3. To illustrate this, suck a milkshake through a narrow straw, then a thick straw. You'll notice a huge difference in flow rate of milkshake into your mouth.
4. This is pretty basic physics, and is included (for example) in the simple physics-for-premed type courses. It tells you about flow rates you can get through various hypodermic needle diameters, etc.