I believe, that burning fire inside a modern tent was tabooed simply because most of them are highly flammable, and that was approved by avid campers because in practice tents fail to keep warm air inside for long. Look at any typical tent, it's simply cannot hold any hot air at all because of abundance of mesh patches. CO2 generated by any fire is very hot, so it will be vented out first through those openings. Camping tents protects from rain, snow, wind, and bugs - that's all.

Personally, I prefer the hammock system for anything above freezing. The secret of warmth at night is your sleeping system (pad + sleeping bag). However, if you want to cook inside the tent (rain or wind outside), or just need to stop uncontrollable shivering - fire your stove inside and don't worry about CO2 (in a 4 season tent, just make sure some of the mesh openings are indeed open). Surely, you should never sleep with any burner on (except for a Catalytic one, dedicated for that), and always make sure there is no chance your burner can be tipped while operating - thats all about fire hazard, not CO2 danger. Also, it is wise to keep any fuel canisters outside after the use, they might leak and poison you.

The easy way to warm up your sleeping bag is to boil enough water, pour it into a plastic bottle, wrap it with some cloth, and hug it after you fully zipped inside the bag, leaving only 1-2 inches opening on the nose level (for breathing fresh air and preventing moisture condensation inside the bag). I'm using the crystallizing salts warmer pad for that, rolled into a tight bundle and placed inside a pair of spare running socks. If shivering is starting (usually in the early morning) - I'm simply bending the activator inside, placing the bundle high between thighs (there are two major arteries there) and having 4-5 hours of cozy warmth. At the breakfast time, I'm boiling that bundle in a cup of water right in the socks to reactivate salts for another night.

Alcohol stoves are quite finicky to operate in a cold weather, they need precise preheating. But they are burning cleaner than gas stoves, producing less CO2. That's why all of the fancy indoors decorative fireplaces are operating on alcohol (liquid or paste) without any chimney or venting requirements attached. For the fuel volume - 1 oz goes for about 5 minutes (in best alcohol burners - 10).