What is the Pasuk referring to when it adds "Achar Shiluchehah"?
Rashi and Rashbam #1: When Hakadosh-Baruch-Hu told Moshe in Midyan to return to Egypt, 1 he took his wife and his two sons and left. However, when he met with Aharon at Har Sinai and told him who they were, and Aharon pointed out that it was painful enough for those who were already in Egypt, and that it was unwise to add any more, Moshe sent them back to Midyan.
Ramban #1, Moshav Zekenim: Having recorded earlier (Shemos, 4:2) that Moshe took Tziporah and her sons with him to Egypt, it needs to mention here that he sent them back, to clarify how they now found themselves in Midyan. 2
Ramban #2: The Torah is telling us that although Moshe sent them back, Yisro now took them out to reunite them with him, since 'one follows the king wherever he goes'.
Seforno: Yisro first sent Tziporah to find out where Yisrael were encamped. 3
Rashbam #2, Hadar Zekenim: When Moshe left the inn (after the incident with the angel), he left Tziporah and her two sons there, 4 and went to Egypt on his own. And now, before traveling to the desert, Yisro picked them up and took them with him to reunite them with Moshe, following which 5 he gave her a Nadan 6 (a dowry). 7
Moshav Zekenim: It is after he divorced her with a Get. Why is she still called his wife? Others cannot marry the widow or divorcee of a king (so it is as if she was still married to him). 8
And when she reported back to him that their next stop would be at Har Sinai, where they were destined to serve Hashem (as Hashem had informed Moshe in Shemos, 3:12), he delayed his visit, and went out only after they arrived at Har Sinai (Seforno).
Since the Torah ought otherwise to have informed us there that Moshe sent them back (Rashbam). Hadar Zekenim