My son (8 yrs.) came back from school today and asked me;
Q. 1) if we eat milchigs on Shavuos because they didn’t have the opportunity to prepare kosher meat on that same day, why wasn’t the milk or cheese prohibited as cholov akum, since when they milked it they were still Goyim? How about the kelim that they used, didn’t they need tevila that can’t be done on Yom Tov?

A. Darkei Teshuva (Y.D. 120) maintains that a Ger should be toivel his utensils when he converts. See also Dovev Meishorim (1: 85) and Tzitz Eliezer (8: 9).
Chok Hamelech (quoted in (Kemotzeh Shalal Rav ”“ Shavuos p. 210) explains that the Ritva and Ran (Avoda Zara 72b) write in the Yerushalmi’s name, that the reason for the immersion of utensils is due to the inherent impurity of the keilim before they entered the Yisroel domain. Therefore. Even if a convert requires tevila for his keilim, in the case of our forefathers, even if they were considered Bnei Noach, before Kabolas Hatora, (Rashi Avoda Zara 3a) they did not have a state of impurity then.
Minchas Aharon (Shavuos) quotes Binyan Tzion (91) that explains that it was permitted to the Bnai Yisroel to observe Shabbos already in Marah, when they were still pre- immersed and not fully converted, because they had already undergone Bris Mila before they left Egypt, and they were not at the stage of Bnai Noach anymore.
He further adds, that Horav S. Z. Auerbach zt’l (Maadanei Eretz 16) maintains that the prohibition to use utensils before tevila is only Rabbinical. It is meant to enforce having the tevila done. However in cases of need, such as one hospitalized or invited when he cannot object, very temporarily, one may use those utensils. And that was the case of the Shabbos of Kabolas Hatora.
(Kemotzeh Shalal Rav, ibid.) adds that the mitzvah of tevilas keilm was only given later on after the battle of Midyan (Bamidbar 31: 23). Similar answers could be given to why they did not refrain from Cholov Akum.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a