I wouldn't worry about it. We had a bantam hen when I was young - she hatched 32 eggs in our feed shed after laying all those eggs herself (and raised over 20 of the chicks to adulthood!). I know one case is not enough to make a rule, but I have certainly heard of far too many instances of hens secreting off and brooding large clutches to believe for a second that an old egg can't hatch. The hens know far better than we do what they are doing. If the eggs don't hatch, then they don't hatch. Not the end of the world.
A bigger concern in this warm weather is that the oldest eggs will hatch a few days earlier than the youngest eggs, so you may have to incubate the youngest eggs an extra couple days or put them under another hen until they hatch. Then you can sneak them back under the original momma at night and she should raise them with the rest of her clutch, even if they are a day or two younger.