Many of the Mefarshim make the points that you have raised. In fact, it would be correct to say that this Gemara -- "They sold him from Shechem, and to Shechem we will return the lost object" -- may be understood more as a Derash about what happened than as a simple explanation of Peshat.

1. However, let us first cite a Midrash which can help us to understand the depth behind what Chazal are attempting to teach us about the burial in Shechem. The Midrash (Shemos Rabah, end of chapter 20) quotes Rebbi Levi who said, "To what can this be compared? -To someone who placed his wine in the cellar. Thieves entered, took the barrel, went away and drank the wine. The owner of the wine came and found those who stole the wine and said to them, 'You drank the wine; you must return the barrel to its place.' Similarly, the brothers of Yosef stole him from Shechem and sold him. When Yosef was about to die, he made his brothers swear, saying to them, 'Please, my brothers, when I was alive you stole me from Shechem, so return my bones to Shechem.'"

2. The commentary of the Maharzav on this Midrash writes that since the brothers should have returned Yosef to Shechem during his lifetime, at least after he dies they should fulfill their obligation to return him to there. The reason is in order to rectify the sin in exactly the same place as they committed it.

(This is in the spirit of the Gemara in Yoma (86b) which states, "Who is a proper Ba'al Teshuvah? One who refrains from doing the sin again when the same opportunity arises once or twice, and he had the opportunity to do the sin at exactly the same place as the first time.")

3. We may add to this that the reason why it was Yosef himself who requested to be buried in Shechem, as the Midrash says, presumably is that Yosef wished to help his brothers achieve a complete atonement for their sin in selling him, and therefore he wanted them to do Teshuvah in the same place that they did the Aveirah.

4. The above Midrash answers your questions, but I will now cite some Mefarshim who make similar comments to yours on this Gemara. The Maharsha here cites Rashi in Chumash (Bereshis 48:22) who says that Yakov promised Yosef that he would receive the extra portion of Shechem, more than his brothers. (The Radak to Yehoshua 24:32 writes that the simple explanation is that since Shechem was in the portion of Efrayim, and it was the first portion that Yakov received in Eretz Yisrael, Yosef was honored by being buried there.) The Maharsha writes that according to this, it is not necessary for the Gemara Gemara here to give the reason that Yosef was stolen from Shechem, but instead it is because Yakov conquered it and gave it to Yosef to be buried there. This is the same as your explanation. Baruch she'Kivanta!

Kol Tuv,

D. Bloom