The answer below is a bit complicated yet I’ve tried to be through yet not make it too hard to follow. But here’s the anatomy and physiology behind the mechanisms of breathing as I’ve been taught as a nurse student.
The result would be the same irregardless of you being able to attain the first two. I’ll explain, even if you can do a complete thought stopping, not having any thought or experience any external stimuli, your body still has an internal stimulus controlled by the central, and autonomous and somatic nervous systems. All these different nervous systems are linked to the brain in some way and in turn do a lot of things, but in regards to your third question.
You see, your breathing is controlled by the breathing center. When you inhale an impulse is sent your diaphragm along with the intercostal muscles in your ribcage, the diaphragm contract, and push your abdomen out and that creates a pressure in your lungs that becomes lower than usual, then to compensate air is sucked to the lung sacks, the air contains oxygen (O2) and Carbon dioxide (CO2) along with a few more things, the lung sacks are filled with air, and oxygen is moved from a place in the lungs out to the blood and then goes to everything in there. When you exhale everything relaxes and air just goes out. This time the amount of CO2 is hoher than the amount of Oxygen. Now with this idea in mind
The breathing center has two chemo receptors, the central and peripheral chemo receptor. The central is located in the liquor in the elongated nerve (medulla oblongata), after blood reaches the brain after going through the body and is filtrated through plexus corideus and into the liquor, it then passes through the central chemoreceptor which will check and measure the the amount of carbon dioxide (CO2) in the blood that’s been filtrated.
And at the same time in the peripheral chemo receptors located in arteria carotis, will check the level of oxygen in your blood. When either of these receptors registers when either of these values are too high or too low, your body will compensate by breathing in different ways. Like in the case of acidosis, when it’s too much H+ in your blood and you will reduce your pH - value as a result making your blood sour and proteins in your body can no longer function as normal and eventually they’ll stop and you’ll die, in that case you have to little H+ meaning your body is alkaline - alkalosis, there will be too little of H+ because the alcaloids will bind H+ and the proteins once again can’t function optimally and their function will eventually seize and you’ll die.
You can easily test this if you want to experience the effect that’ll eventually lead to respiratory acidosis hold your breath for as long possible and see what happens. Or rather respiratory acidosis usually happens when you hypoventilate.
If you want to experience the other thing - respiratory alkalosis, you hyperventilate, you should become very light headed and dizzy if you do it for a long time, alkalosis may happen to you.
The limit is a human supposedly can hold their breath before passing out is around 2 minutes beyond that once you pass out, you’ll die unless you are revived. So in a nutshell, the cells in your body needs oxygen to function and to keep you alive, without you die, one way or another.
Sorry about the technical stuff, and to those who’ve studied this, sorry about the over simplification, and I might have missed details, but as far as I can tell this is the general idea.