Eggs will float as they approach hatching, as the gas exchange is different compared to how it was during development. The shells get more friable as the membranes alter in the eggs. I have had hens abandon eggs during the coldest parts of winter even at late stages of development, and still the chicks will hatch. They become dormant during excess cold, the only time it'll kill is when they're chipping and expending their last amounts of available energy and damaging the shell so are unable to go back into 'sleep/suspended' mode, so to speak.
Of course, remaining without any heat for a good few days will kill them too, but they're quite remarkably able to survive that too. I think they have some ability to regenerate from early damages sustained from maternal management faults, such as yolk-sticking, etc, if the egg's restored to healthy management within three or so days. I've had very bad mother hens whose nests I've found out in the forest, who've had eggs weeks old that have not died nor developed, which are viable once put under a hen who sits properly. They can survive amazingly long lengths of time when only receiving half an hour's heat a day, or less; just enough to keep the cells from dying, but not enough to foster proper development. Of course, it decreases hatching rates, it takes a very good egg to tolerate that treatment. But I don't give up on eggs too quick, because I've given up too quickly in the past, going by the books, and there's few feelings as bad as realizing you just wasted a viable baby.
Concerning dead-in-shell chicks, have you heard of 'swimming' them? I've not tried it but apparently it often works. You could find better walk-throughs on it, but basically you hold the chick (once you've hatched it) in such a way as to keep its beak above water and support its neck/body, and you drag it through warm water. People say it 'kickstarts' a lot of dead but full term babies, and I'd believe that since I've 'jump-started' a few recently dead animals in my time, and baby chooks are incredibly tough yet simple.