From Explaining the Customs of Bris Milah (relevant excerpt):
WHY IS THERE A CHAIR FOR ELIYAHU?
According to the Midrash, Eliyahu Hanavi attends every bris. Before
Eliyahu rose to heaven and assumed the role of an angel, he was the
prophet responsible for admonishing the wicked monarchs Achav and
Izevel. Eliyahu was a zealot for Hashem’s honor (...
You could start with this artscroll book: Eliyahu Hanavi: The prophet through the prism of Tanach, Talmud and Midrash by R Avraham Yom Tov Rotenberg which artscroll describes as
Who was Eliyahu? How can we understand
his zealotry? What was his mission? How was he unique? What is his
legacy for us? The Sages in the Talmud and Midrashim, the ...
I've seen a couple of sefarim bring down from a commentary called "Mekor Chesed" to Sefer Chasidim that "zachur latov" is based on the pasuk in I Kings 17:18, where the woman feared that Eliyahu was coming to cause her to be remembered for the bad, that her sins would be recalled. We therefore pray that when Eliyahu comes it should be for the good and that ...
TL;DR the source that Eliyahu "drinks" from each cup at the seder can be (at most) traced back a few hundred years
An OU.org article titled "Eliyahu HaNavi at the Seder" adresses this topic.
Before we determine whether Eliyahu Hanavi drinks from every kiddush cup designated for him at the seder, does he even even APPEAR at the seder?
The article ...