Did you manage to take any data Graham, or was it the fact that the tent felt warmer?.....
Isn't that enough?
To answer the OPs question. I can find no recorded death online due to CO from candles, either multiple or single. This does not mean it is safe......
Yeah, my only real fear would be fire itself.
I've been out in that desert, just north of Tombstone....the wranglers wouldn't even let me smoke a cigarette! Too be fair it was very very dry
LOL. Exactly!
Agreed, the flame of a tealight candle will flicker and die before you do.
Sure there are better alternatives to a candle for more heat in a tent (better sleeping bag, zippo hand warmers, Jennifer Connelly...) but candles will work.. In fact candles are excellent to use in primitive debris shelters (because candles are common place in 'survival tins' and debris huts are well ventilated)....
I'd think twice about the debris shelter though; again mainly because I'd be afraid of the fire danger, not CO poisoning.
Don't rely on piezo ignition for stoves or lanterns, they are very prone to failure, pack a couple or three Bic lighters to be on the safe side.
Never had one fail yet, but yeah, I do carry matches and/or lighters.