Why did Eliezer attribute his refusal to his success?
Rashbam: Hashem made my way successful. If there is a delay, this is not success.
Ha'Emek Davar #1: If a messenger failed, he is pleased to delay, so the sender worries what happened to him on the way, and when the Shali'ach returns, he is happy that he returned and is not so pained that the mission failed. If he succeeded, there is no reason to delay
Malbim, ha'Emek Davar #2: Since Rivkah came immediately, miraculously, this shows that not only the match is from Heaven, but also Hashem fixed the time for now. 1 To stay here would be delaying the time that Hashem fixed.
See, for example, how he had arrived in Charan on the same day that he left Chevron. (EC)
Why did Eliezer repeat "Shalchuni lA'doni", but he added "v'Elchah"?
Ha'Emek Davar: "Shalchuni lA'doni" includes if Rivkah would leave their house and remain in the city (e.g. a year or 10 months), and Eliezer would guard her there. He replied that they should send him to return immediately.