All very interesting!

Here is a breif outline of breath holding physiology, more later.

Firstly, in healthy individuals, breath holding has nothing to do with the amount of oxygen in your body (surprising I know).

The body senses levels of carbon dioxide (product of respiration) in the blood. High levels cause us to breathe (way before levels of oxygen reach dangerous levels, unless you hyperventilate first which lowers the residual level of co2 before you start).

You can improve your breath hold in two ways: 1. slowing the production of co2 (get fitter), 2. resist the stimulous to breathe (practice/will power).

This is way oversimplified but is the basics that matter.

as for training with low levels of oxygen, this is a bit more complicated.
hypoxia (low o2) stimulates the body to prduce more red blood cells, which increases the oxygen carrying capacity of the blood and therefore decreases the work of the heart (because oxygen delivery = volume of blood * amount of o2 in the blood). Decreased work of the heart leads to less production of co2 and therefore longer breath hold.

all interesting!
ps. cool website!