Eggs will remain viable at 70 degrees or above... I suspect they may form a few cells, but incubation takes place at 99.5-100 degrees. I suspect there will be a few cells dividing at 70 (or 78) but you aren't going to have a chick in there, cells at that point are basically microscopic. If you're THAT worried, put them in the fridge for goodness sake!
All we can do we is tell you what we do, as I said, mine on the counter are fine for around a week, plus or minus a couple days I'd say... and I don't have air conditioning at all. My basement is cool, I'd say at least slightly below 70 even in the summer, and probably below 60 in the winter. I really need to put a thermometer down there... in fact, I'm doing that now. It sounds like you've already been leaving yours on the counter, no horror stories yet? Then chill mama! (said I was old, not ancient!
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(no maliciousness or spite intended, I'm chuckling gently over it, never laughing at people I promise!