Why is Shlissel Challah permissible?
In ancient times, the Roman pagan holiday of Saturnalia included a practice of baking some type of cake with a fava bean hidden inside, with a prize for whoever got the piece containing the bean. In medieval times, the prize often included being the ruler of the region for the year, hence why this practice is called the “King’s Cake.”
Over time, the custom became distorted by its adoption into Christianity: rather than a winter custom, the practice was near-universally shifted to the spring and practiced during Epiphany (during Mardi Gras), the day believed to be the conception of their Messiah. Further, the specific object which was hidden (called a fève, after the original fava beans used) changed over time and locale, broadening to whatever fruits were in-season (almonds, dried fig pits), candies, figurines, and...keys.
It seems pretty straightforward to suggest that the Jewish custom of shlissel challah, where on the first Sabbath after Passover a key is baked in the challah (or in a more recent take, shaping the challah itself as a key), originated from this practice.
The Torah commands us not to emulate the practices of the idolatrous nations (Leviticus 18:3 et. al.; Rambam, Hil. Avodah Zarah 11:1ff). Why would Shlissel Challah be permissible?