I'm not sure about the ones that are in storage before V3 happened.. But you can't store eggs anywhere else. They have to stay in your hatchery. Seasonal eggs turn old in about a year, but I'm not sure how long it'll be until they spoil.
My advice is hatch eggs as soon as they spoil. Or continue hatching like you are now and hope that you'll get a Mutant creatu, regardless of egg age.
You'll actually have to plan your hatches if you want to hatch every egg you acquire.