You can try to save some in case she goes broody but I agree an incubator gives you some control. Waiting on a broody you are totally at her mercy.
In the last part of the mating act the hen stands up, fluffs up, and shakes. This gets the sperm into a special container where it can remain viable for nine days to over three weeks. It can vary that much by individual. That difference in how long that sperm remains viable makes this risky. So she may lay a fertile egg anywhere from just under two weeks to over three weeks. The longer it goes the riskier it gets.
That embryo remains viable after the egg is laid. How long depends on how the egg is stored. One week isn't that bad. Under ideal storage conditions two weeks with good results is doable. After that it gets iffy.
If you follow this link it tells you what those ideal storage conditions are. There are three that I'd pay close attention to. One is temperature. The closer you can store them to 55* Fahrenheit the better. Most of us don't have anything that close. Try to avoid heating and cooling cycles too, a steady temperature is best.
Moisture control. The longer an egg is stored and the drier the conditions the egg is in the more moisture it looses. They should be stored in a high humidity environment. Most of us don't have that, any cooled area is probably low humidity. That's why the suggest the plastic wrap, to hold moisture in.
The other really important one is turning. Turning does different things but at the storage stage the big benefit is to keep the yolk from settling and touching the inside of the egg shell. If it touches it can get stuck and the chick cannot hatch.
https://aggie-horticulture.tamu.edu...e-Cartwright-Incubating-and-hatching-eggs.pdf
I don't know how many eggs you plan to put under that tiny Serama if she goes broody. She needs to be able to comfortably cover them all. That might be as few as four. You never know how many will hatch or what sex they will be if they do hatch. There are no guarantees you will get any pullets. The more eggs you hatch the better chances for a pullet. That's why I suggest four instead of two.
If you crack an egg you can look for the bull's eye. You obviously cannot hatch that egg but if one is fertile the others probably are. This link shows you what to look for.
Fertile Egg Photos
http://www.backyardchickens.com/t/16008/how-to-tell-a-fertile-vs-infertile-egg-pictures
You have decisions in timing to make. You can start saving and storing the eggs so you have them ready to go. Mark the date they are laid and rotate them, keeping the freshest and eating your oldest. When that cockerel is gone, keep saving eggs as she lays them, I'd stop just over two weeks. Then store them in the best conditions you can in hopes you get a broody hen.
If hatching her eggs means that much to you get a cheap incubator and try.
