Homer nowhere mentions any sacrifice of Iphigenia, and debate is ancient on whether the “Iphianassa” whom Agamemnon offers as a potential bride to Achilles, at Iliad 9.145 & 9.287, is the same person. Since nowhere have I found any hint that Achilles was necrophiliac, this offer clearly implies that the daughter in question was still alive so far as her father knew, nine years into the siege.
The Agamemnon of Aeschylus has the chorus remember the sacrifice of Iphigenia as completed, with the victim unwilling, bound and gagged lest an outcry from her bring misfortune. Throughout the Oresteia revenge for Iphigenia is represented as Clytaemestra’s primary motivation and justification for murdering her husband, with the concubinage of Cassandra just an insult added to injury, and hardly a justification for the act of a queen who was adulterous herself. (Bronze Age culture both in Greece and in Israel appears to have seen sexual promiscuity as culpable only in the female marriage partner.) The purpose of the sacrifice was to calm a persistent adverse wind from Thrace, understood as expressing the displeasure of Artemis. Agamemnon himself is said to have “plunged [his neck] into the yoke-strap of Necessity” [ἀνάγκας ἔδυ λέπαδνον] in this matter, a highly ambiguous phrase in relation to his willingness to go through with it, though that part of the choral ode goes on to make him out as willing.
In Euripides’ Iphigenia in Aulis the ostensible purpose of the sacrifice is to bring a wind; the fleet is stuck for lack of any wind, not an adverse one, though the thousand ships are usually understood as having oars. The original ending to the play appears to be lost; what we have is a Deus ex machina rescue of the sacrificial victim, with animal substitution as in Genesis 22.11–13. The lost ending was most likely similar: Euripides was notably fond of the Deus ex machina, and had earlier written Iphigenia among the Taurians premised on the title character’s having survived her ordeal at Aulis, not that that precedent would have been binding on him. (For this and other issues with the text, see the introduction to the Aulis play in the newer Loeb edition of Euripides by David Kovacs.) Earlier in the Aulis play, in what Aristotle hailed as a prime example of character growth and transformation in tragedy, Iphigenia had so far reconciled herself to the sacrifice as to become the willing victim that sanctity normally required.
In Euripides’ Iphigenia in Aulis, and even more prominently in Michael Cacoyannis’ splendid 1977 film version of it, doubt is cast on whether there even are any gods (thanks, Protagoras), let alone a god who is demanding this utterly unheard-of and counter-cultural act. (This film, titled simply Iphigenia, or ΙΦΙΓΕΝΕΙΑ, is the source of most of the grainy moving images in the execrably snarky YouTube video linked from OP’s question. The film ends ambiguously, though most viewers interpret it as implying that the sacrifice was indeed carried out, with no miraculous animal substitution, even though by that point a wind has started to blow.) It may well be just a plot cooked up between the wily and unscrupulous Odysseus (perhaps hoping for an early ticket home if Agamemnon refuses to go through with the sacrifice) and the seer Calchas. By the end of the play and film, both Agamemnon and Menelaus have spoken against the sacrifice, joining Clytaemnestra and even the boy Orestes in opposition to it, but despite this family unanimity resistance is seen as futile, given the blood-lust of the army, which Agamemnon terms “the many-headed monster I command.”
So ancient sources are kind of all over the map on this sacrifice, as on so many matters of myth; but throughout it is clear that human sacrifice is completely abnormal in ancient Greek/Mycenaean culture, as it was in ancient Hebrew culture.
As for Tantalus’s being in “Hell,” he is indeed shown in Odyssey 11.582–92 as punished in the afterlife, though there is no separate afterlife-world for such punishments; but how would his great-grandsons know this years before Odysseus’ visit to the realm of the dead? Did Heracles somehow report it out after his twelfth labor?