Use of שלח – The opinions in the Mekhiltas link שלח here with its use in the context of divorce in Devarim 24:1-4. See שלח for similar cases, and for a discussion of the terms used for divorce in Biblical and Rabbinic Hebrew.
Chronology – See Zipporah's Return to Midyan for the different possibilities as to when the divorce could have taken place. The difficulty according to all of the variations, though, is that the Torah makes no mention of a divorce until Shemot 18.2
Yitro's initiative – If Moshe divorced Zipporah, why is Yitro bringing her back, and why does he still refer to Zipporah as Moshe's wife in 18:6? R. D"Z HoffmannShemot 18:2About R. D"Z Hoffmann asks these questions and suggests that perhaps the divorce was only a formality intended as a temporary measure until Moshe would be out of danger.3
Motivation – It is possible that this position is motivated by a desire that Moshe not be married to the daughter of an idolatrous priest when he becomes the leader of the Children of Israel – see Moshe and Zipporah's Marriage. If so, it is possible that Moshe never remarries Zipporah. See also Miryam's Critique of Moshe and his Cushite Marriage for the various positions regarding Moshe's marriage to (or divorce from) the Cushite woman.
Moshe Only Sent Zipporah Home but Did Not Divorce Her.
Chronology – See Zipporah's Return to Midyan for the different possibilities among these exegetes as to when Zipporah was sent home. According to R. Saadia, Shemot 4:20 already tells of Zipporah returning home, while according to the other commentators the Torah notes this information only (parenthetically) in Shemot 18 and not when the event transpired.5
Zipporah Sent Messengers
שִׁלּוּחֶיהָ describes Zipporah's sending (שלח) of messengers to find out where Moshe was encamped. According to this approach, Zipporah is actively seeking to reunite with Moshe, and Yitro is attempting to assist her. See Yitro's Purpose.
Chronology – According to this opinion, the narrator is recounting an event which is happening now (rather than previously), and it is clear why it is recorded here.7
Zipporah's initiative – As Yitro takes the initiative throughout the entire chapter and Zipporah takes a backseat, it is surprising that it would be Zipporah who sent the messengers and not Yitro himself.
Dowry or Gifts
שִׁלּוּחֶיהָ refers to a dowry given by Yitro to Zipporah or gifts sent by Zipporah to Moshe. According to all of the commentators who adopt this possibility, the word is linked to the two other Biblical appearances of the noun שִׁלּוּחִים. However, they disagree regarding who gave what to whom and as to when this happened. These questions greatly depend on whether the term שִׁלּוּחִים can describe any gift or only a dowry – see שלח for the basis of the dispute on this matter.
Yitro Gave Zipporah Now
Yitro gave Zipporah her dowry now since he was returning her to Moshe.
Chronology and foreshadowing – Rashbam explains that he prefers this opinion because were the verse to be referring to an event which took place earlier such as Moshe's sending away of Zipporah, this event would have been noted already at the time when it happened,8 in order to lay the necessary groundwork for our verse.9
The meaning of אַחַר – Even though it would appear from the simple reading of the verse that the dowry preceded the taking of Zipporah, Rashbam (because of his assumptions above about chronology and foreshadowing) is forced to explain that the dowry came only afterwards. He thus interprets אַחַר as "and afterwards…" rather than "after."10
Yitro Gave Zipporah Previously
Yitro previously gave Zipporah her dowry11 when she married Moshe.
Chronology – R. Yosef Bekhor Shor reads all of 18:2-4,6 as a retrospective summary of Yitro's prior actions and agreement with Moshe.12 According to him, Zipporah initially accompanied Moshe to Egypt, anticipating that they would only be there for a short time. However, when Paroh caused the Exodus to be delayed,13 Yitro traveled to Egypt to bring Zipporah back to Midyan14 so that she could manage the properties given to her as part of her dowry, and this is what is described in 18:2-4.
Yitro's initiative – According to R. Yosef Bekhor Shor, when Yitro came to collect Zipporah from Egypt, he promised Moshe that after the Exodus, he would meet up with him and return Zipporah to him, and it is this pledge which is recorded in 18:6. For elaboration, see Purpose of Yitro's Visit.
The meaning of אַחַר – In order to facilitate his explanation, R. Yosef Bekhor Shor interprets אַחַר as "nearby" (סמוך),15 i.e. Zipporah had returned to Midyan to be near her estate.
Zipporah Gave Moshe Now
Zipporah sent gifts to Moshe immediately prior to her arrival.
Chronology and Zipporah's initiative – Zipporah is sending ahead a gift before she arrives to find favor in Moshe's eyes.17 According to this approach, the events are being told when they happened.
The meaning of שִׁלּוּחִים – According to these exegetes, the word שִׁלּוּחִים means any gift which is sent,18 and not specifically a dowry given when a father sends away his daughter to her husband's home.19
According to all of the above approaches, and regardless of how one explains אַחַר שִׁלּוּחֶיהָ, it is clear from Shemot 18:1-6 that Zipporah was sent home at some point. For an analysis of the various positions on the timing of this event – see Zipporah's Return to Midyan.