I like to sleep under quilts, blankets and sheets even when the weather is warm. I never pull them all the way over my head though, because I am afraid of smothering. Should I continue to leave an air hole 
or could I still breath if I slept buried under a quilt or two? (I am in good health with no breathing problems.)
I do the same thing. If I’m uncomfortable in my sleep or need mre air, I pull the blanket down withoput even waking up. You’re in no serious danger. I hope.
My experience is that you will unconsciously remove your blankets if they are causing trouble. I frequently find that I’ve kicked off my covers because of overheating.
Unless you’re rolling up in your blankets to form a tight cocoon that binds your arms, then sticking your head under your pillows, I don’t think you have anything to worry about.
I will add that my sister’s chihuahua has been known to sleep under the covers against her side, completely buried, with no problems. That’s under multiple blankets.
You could split the difference and buy a thermal blanket. They’re like yarn with a wide, porous weave, but very warm nonetheless. No smother danger there! But AFAIK, no one has ever smothered under a good old-fashioned quilt.
IMHO, no, nope, not ever. OTOH, I have a hard time seeing how anyone could suffocate that way anyhow. I remember way back, about the tenth time I’d seen the old guy-in-a-hospital-bed, evil-person-smothers-them-with-a-pillow thing. I couldn’t believe it even then; I tried really hard to smother myself with a pillow, but decided that unless it was stuffed with plastic shopping bags, there was no way it was gonna work. 