Why are sin offerings necessary in 43 & 45? This seems contrary to the idea that Jesus is the final sacrifice.
I've read a few different interpretations such as memorial sacrifices, or non literal, but I'm really struggling to understand this.
The common rush to understand a passage often means that the interpreter rushes to the application before understanding the simple literal meaning. The section of Eze 39-48 is a very common example of this.
Literal Text Meaning
The intention of this last section of Ezekiels' writings is actually givenm in in the last part of Eze 39, for example, look at V25 -
Therefore this is what the Lord GOD says: Now I will restore Jacob from captivity and will have compassion on the whole house of Israel, and I will be jealous for My holy name.
Therefore, Eze 40-48 is a prophecy of a restored Jerusalem temple following the Jews' return from Babylonian captivity. Unfortunately, the returned Jews did not return in the numbers that were hoped for and were not as faithful as they should have been (See the book of Nehemiah!)
Thus, the prophesied grand temple was never built; the sacrifices as predicted were never implemented; and when the "glory of the LORD" (Eze 43:1) was to come to this new temple, ie, the Messiah as king of restored Israel, He should have offered the prescribed sacrifices listed in Eze 34, 44, etc.
However, as recorded in the NT - the Jews were not faithful and Jesus, instead of coming to the temple and offering sacrifices, was rejected.
Matt 23:35-29 - And so upon you will come all the righteous blood shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Berechiah, whom you murdered between the temple and the altar. Truly I tell you, all these things will come upon this generation.
O Jerusalem, Jerusalem, who kills the prophets and stones those sent to her, how often I have longed to gather your children together, as a hen gathers her chicks under her wings, but you were unwilling! Look, your house is left to you desolate. For I tell you that you will not see Me again until you say, ‘Blessed is He who comes in the name of the Lord.’”
Thus, the Jews and the Jewish temple were rejected because of their unfaithfulness. (See general principle in Eze 18 and Jer 18 about conditional prophecy.)
However, that does not prevent the Israelite sacrifices having some significance beyond the old Israelite economy as it pertains to the "New Covenant". See appendix below.
APPENDIX - Levitical Covenant Significance
Levitical Covenant: – Lev 1-9, 16, 21-27 , Num 3, 4, 8, 18, 25:10-13, Deut 33:8-11, Neh 13:29, Mal 2:4-8. This is an eternal covenant (Num 25:12, 13, Ps 106:30) of salt, Num 18:19.
The Levitical covenant is stated in Num 3:11-13, 25:12 where God takes the Levites (especially the line of Phineas as High Priest) instead of the all the firstborn of each family, thus changing the (informal) priesthood from the firstborn of each family to the (formal) priesthood of tribe of Levi. Several Bible writers refer to this Levitical Covenant including:
Jesus is the high priest of the New Covenant Heb 4:14-16, 5:10, 7:23-28, 8:1, 2, 9:1-28, 10:1-18. Thus, Jesus inherits and was the fulfilment of the Levitical Covenant (See Mal 3:1). Jesus did this in several ways as well such as:
Jesus did not abolish the Israelite Covenant and Moral Law but came to fulfil them, Matt 5:17-20, Gal 3:21. The moral law still applies to Christians BUT Jesus fulfils the Levitical and Davidic covenants by being the eternal High Priest and the eternal King of the Kingdom of Heaven.
Ezekiel's prophecy was made prior to the building of the Second Temple. The reason it includes sin offerings is that Ezekiel did not understand his prophecy to refer to the messianic age as Christians think of it, but to the age of the restored Temple, which occurred in the time of Ezra and Nehemiah.
Being himself a priest, Ezekiel conceived of the Temple and its priesthood as central to Judah's restoration after its exile in Babylon. Judah (sometimes called Israel) was to become a literal "kingdom of priests and a holy nation." (Exodus 19:6) Ezekiel's prophecies are not messianic in the sense that term is used by Christians today. The savior of the nation is not Son of God or any a human being, but is God himself. Here, savior's role is not save people from sin but to bring the people of Israel home:
I myself will search for my sheep, and will seek them out. As a shepherd seeks out his flock when some of his sheep have been scattered abroad, so will I seek out my sheep; and I will rescue them from all places where they have been scattered on a day of clouds and thick darkness. And I will bring them out from the peoples, and gather them from the countries, and will bring them into their own land; and I will feed them on the mountains of Israel. (34:11-13)
One may ask: then how was Ezekiel's prophecy fulfilled, since many of the details of his ideal Temple did not become realities when the Second Temple was finally built? To resolve this issue we need to concentrate on the broad sense of the vision and not its minute details. As Britannica puts it:
In contrast to those hoping for national restoration under a Davidic king, Ezekiel envisaged a theocratic community revolving around the Temple and its cult as the nexus of the restored Jewish state.
God did bring his people home, and they did restore the Temple. So the core of Ezekiel's vision was indeed realized, partly under the rule of Ezra and later during the Hasmonean period after the Maccabean revolt. Some commentators add that Ezekiel's vision for the ideal Temple could have been fully realized by Zerubbabel (a descendant of David who ruled as governor of Judea) immediately after the return from exile, but this plan was not realized.
What are you, O great mountain? Before Zerub′babel you shall become a plain; and he shall bring forward the top stone amid shouts of ‘Grace, grace to it!’” Moreover the word of the Lord came to me, saying, “The hands of Zerub′babel have laid the foundation of this house; his hands shall also complete it. Then you will know that the Lord of hosts has sent me to you. (Zech. 4:7-9)
Ezekiel's prophecy includes sin offerings and other practices rejected in Christian tradition, because he was not envisioning a Temple of the messianic age, and his focus was the restoration of true worship as specified in the Law of Moses.