To review, our problems are two:
1) Rashi has taken a midrash which the Sages attached to one verse, and applied it to another. Why?
2) The comment under discussion addresses a spelling mistake, suggesting that this error was made deliberately by the author of the Torah (ie: The King of Kings) for the purpose of conveying an indiscretion on the part of Abraham's servant. But if this is so, why not let us know about the indiscretion when it was committed, rather than later, as the servant retells the story?
As Lkwd and Chaim told us in the previous comment thread, the Midrash was written to explain the servant's use of the word "oo-lie," instead of "pen." Both words mean "perhaps," but "oo-lie" carries the sense the outcome is desired ("pen" is closer to the archaic "peradventure.")
The role of Midrash (generally, with many exceptions) is to give us back story, to fill in the blanks and to connect the dots. At the time the midrash was written, a typical reader would have noted the use of the word "oo-lie" instead of "pen" and instantly recognized that Eliezer wanted his mission to fail. But the text gives no explanation for this strange desire. Enter the midrash, with the story of Eliezer's daughter.
Rashi's role (generally, with far fewer exceptions) is to explain problems in the text. When Eliezer, in the study, first says "What if the woman is unwilling to come back with me to this land? Shall I then take your son back to the country you came from?" there is no problem with the text. The word "oo-lie" -and its implications - are perfectly clear. There are no misspellings, and no reason for Rashi to say anything. So he doesn't. Later, when the spelling error appears Rashi draws on the midrash and tells us why the word is misspelled: To let us know that Eliezer wanted Isaac for his own daughter.
This isn't a question on Rashi, but on the Torah itself. Why was Eliezer's secret wish indicated at the end of the story, rather than the beginning? The most satisfying explanation comes from the Chizkuni, who says that the servant purposely used the word "oo-lie" as he retold the story because he wanted to make it clear and obvious to Rivka's relatives that he wanted Isaac for himself. This was part of his sales pitch, the Chizkuni suggests, his way of making it clear that Isaac was a desirable son-in-law. Had the Torah misspelled the word "oo-lie" the first time it appeared, we may have judged Eliezer unfavorably. By misspelling the word the second time it appeared - when Eliezer was not thwarting the mission, but advancing it, and at the expense of his own interests - the Torah tells us something good about Eliezer's character and his loyalty to Avraham.