When Did Zipporah Return to Midyan?

Introduction

Shemot 4:20 states:

"Moshe took his wife and his sons, and set them on a donkey, and he returned to the land of Egypt."

From this verse it would seem that Moshe took Zipporah and his children with him back to Egypt. However, it is clear from Shemot 18 (regardless of how one explains אַחַר שִׁלּוּחֶיהָ) that Zipporah was back in Midyan and not at Moshe's side at the time of the Exodus. Various exegetes attempt to reconcile the stories in these two chapters, and they present a spectrum of opinions as to when Zipporah returned to Yitro's home:¹