There is also such a thing as the instinct of self-preservation, I believe that it will work even before the loss of consciousness occurs! That is, when you are close to losing it, you will sigh reflexively, I personally think so.
No, it won't. Before passing out - yes, as the brain will not have enough oxygen. But immediately after passing out, you will immediately begin to breathe, because otherwise you will die. Yes, nature has foreseen everything.