You are asking an intriguing and insightful question. I am quite impressed that you dug down deep enough into Ezekiel 40-48 to find the reference to a sin offering in Ezekiel 45:18-21. First of all, we need to ask what is this temple in Ezekiel 40-48 and what is God’s purpose for putting it in the Book of Ezekiel. To answer this, I must do some speculating, as the Bible does not give a direct answer, but, of course, this should not be mere speculation. Our explanation should fit the content and the context of Ezekiel. Below I will be speaking as if what I am writing is “the” answer, but please take what I say for what it is worth and evaluate for yourself.
I believe that the temple in Ezekiel 40-48 is an idealized temple. It is a picture of the ideal temple, both to the Jews and to Christians, who are bound to see it fairly differently. For the Jews it is an idealization of the temple in Jerusalem. For Christians it is a picture of the new heaven and the new earth in which we will dwell with Jesus. It is worth noting that this particular temple as described in Ezekiel was never built. The temple that was completed in 516 BC under Zerubbabel was not built according to this pattern. Neither was the rebuilt temple under Herod the Great. To build a Jewish temple on the temple mount in Jerusalem at this point would set off world war III. This will not be happening. When I look at the temple in Ezek 40-48, I look at it through New Testament eyes. To me it is a picture of heaven, but written using entirely (or almost entirely) Jewish elements. There is an almost insane level of detail here to tell us about the holiness and the glory of God. “Son of man, describe the temple to the people of Israel, that they may be ashamed of their sins. Let them consider its perfection.” (Ezekiel 42:10) I believe that this verse captures what is going on with the Ezekiel temple. I have written a book titled From Shadow to Reality (www.ipibooks.com) in which I describe in much detail the symbolism of the prefigures and types of the objects in the temple and in the sacrifices in the temple. In Old Testament foreshadows, the priests are us. We are a royal priesthood and a holy nation. (1 Peter 2:9). The rooms for the priests (Ezek 42) are prepared for us. In Ezek 43 the glory of the Lord returns to the temple. The Jews would have seen this as a prophecy of the return of God to his temple in Jerusalem in 516 BC when the glory filled the temple again. I do not disagree with this interpretation, but believe that God principally has in mind the heavenly tabernacle of which the earthly one is merely a foreshadow. (Hebrews 9:1-8). The priest being restored, and especially the priests of Zadok, are a prefigure of us as Christians serving in the heavenly tabernacle. “They are to teach my people the difference between the holy and the common.” (Ezek 44:23) Again, this passage has relevance both for the Jew and for the Christian. Moving forward, in Ezekiel 47 we have a picture of God’s people restored, not to the the idealized Holy Land, but the symbolism actually comes from the Garden of Eden in Ezek 47:1-12. This is a beautiful symbolical picture of the New Heaven and the New Earth.
Now it is time to talk about the grain offerings, the burnt offerings, the fellowship offerings and the sin offerings in Ezekiel 45-46. As with the other parts of the vision, I believe that there is a dual fulfillment, both as an idealized Jewish temple which was never built, and as a picture of a future Kingdom of God. For us, Jesus is our sin offering, and our guilt offering. (Hebrews 8:3-6, 9:11-14, 10:1-18). He is the perfect high priest (Hebrews 4:14-5:10). The vision in Ezekiel 40-48 is of a future perfect temple–a heavenly one–in which we will serve as priests and in which Jesus will be our high priest. We will not be offering literal grain, burnt and sin offerings, but we will be offering the New Testament equivalent of those things (see my book, mentioned above for more details on the fulfillment of Old Testament sacrifice foreshadows in the New Testament).
So, my answer is that if we look at Ezekiel 40-48 through New Testament/Christian eyes, the sin offering there is Jesus himself. The Jewish readers of Ezekiel before Christ would see this differently and this is not a “wrong” interpretation. As for figurative versus literal interpretation, I would note, as I said above, that this temple was never built and it is my opinion that it will never be built by human hands on the earth. I suppose, then, that I take the “figurative” view. Honestly, I do not know what the literal view would be. Might this be the premillennial theory that Jesus will literally come back to the earth and dwell in a reconstructed temple, offering Jewish-like sacrifices? I believe that this is a gross misunderstanding of this passage. The need for such sacrifices was “old and fading” at the time Hebrews was written, and since then it has “disappeared.” (paraphrasing Hebrews 8:13). This idea of a reconstituted Jewish form of sacrificial worship by Jesus in a physical temple in Jerusalem is a mistake on several levels and should be rejected.