Yeah, single combs pop up in wyandottes all the time. My only wyandotte so far was a silver laced pullet from a hatchery and she had a single comb too.
Absolutely. If the breeds listed on the hatchery website were used when your OE hen was hatched then she would have definitely inherited a single comb gene from at least one of her parents (she actually looks to have a modified single comb herself). Since single combs are recessive it means your...