In The IVP Bible Background Commentary, Craig Keener notes that the language of “Never before or again” is sometimes used hyperbolically in the Bible. He indicates this can be seen when you compare Jos 10:14 with Ex 8:13, Nu 14:20, and 2 Ki 6:18 (cf. also 2 Ki 18:5 with 2 Ki 23:25).1
I would suggest that Yahweh’s words in Eze 5:9–10 should also be understood hyperbolically. Viewed from this perspective, the purpose of his words would be to emphasize the severity of the punishment that would come upon the people of Judah rather than to inform them that it was a form of punishment he would only use once. This interpretation seems to align better with the fact that when Eze 5:9–10 was fulfilled—presumably during the siege of Jerusalem by Nebuchadnezzar (ca. 587/586 BC; cf. La 4:10)—it was neither the first nor last time that Yahweh would cause Israelites to eat their own children. Yahweh had already caused this to occur during the siege of Samaria by Ben-hadad (ca. 9th century B.C.; 2 Ki 6:24–29), and he caused it to happen again during the siege of Jerusalem by the Romans in AD 70. The Jewish historian Josephus provides the following account from the latter siege:
4. (201) There was a certain woman that dwelt beyond Jordan, her name was Mary; her father was Eleazar, of the village Bethezub, which signifies the House of Hyssop. She was eminent for her family and her wealth, and had fled away to Jerusalem with the rest of the multitude, and was with them besieged therein at this time. (202) The other effects of this woman had been already seized upon; such I mean as she had brought with her out of Perea, and removed to the city. What she had treasured up besides, as also what food she had contrived to save, had been also carried off by the rapacious guards, who came every day running into her house for that purpose. (203) This put the poor woman into a very great passion, and by the frequent reproaches and imprecations she cast at these rapacious villains, she had provoked them to anger against her; (204) but none of them, either out of the indignation she had raised against herself, or out of the commiseration of her case, would take away her life; and if she found any food, she perceived her labors were for others, and not for herself; and it was now become impossible for her anyway to find anymore food, while the famine pierced through her very bowels and marrow, when also her passion was fired to a degree beyond the famine itself; nor did she consult with anything but with her passion and the necessity she was in. She then attempted a most unnatural thing; (205) and snatching up her son, who was a child sucking at her breast, she said, “O, thou miserable infant! For whom shall I preserve thee in this war, this famine, and this sedition? (206) As to the war with the Romans, if they preserve our lives, we must be slaves! This famine also will destroy us, even before that slavery comes upon us:—yet are these seditious rogues more terrible than both the other. (207) Come on; be thou my food, and be thou a fury to these seditious varlets and a byword to the world, which is all that is now wanting to complete the calamities of us Jews.” (208) As soon as she had said this she slew her son; and then roasted him, and ate the one half of him, and kept the other half by her concealed. (209) Upon this the seditous came in presently, and smelling the horrid scent of this food, they threatened her, that they would cut her throat immediately if she did not show them what food she had gotten ready. She replied, that she had saved a very fine portion of it for them; and withal uncovered what was left of her son. (210) Hereupon they were seized with a horror and amazement of mind, and stood astonished at the sight; when she said to them, “This is my own son; and what hath been done was mine own doing! Come, eat of this food; for I have eaten of it myself! (211) Do not you pretend to be either more tender than a woman, or more compassionate than a mother; but if you be so scrupulous and do abominate this my sacrifice, as I have eaten the one half, let the rest be reserved for me also.” (212) After which, those men went out trembling, being never so much affrighted at anything as they were at this, and with some difficulty they left the rest of that meat to the mother. Upon which the whole city was full of this horrid action immediately; and while every body laid his miserable case before their own eyes, they trembled, as if this unheard-of action had been by themselves. (213) So those that were thus distressed by the famine were very desirous to die; and those already dead were esteemed happy, because they had not lived long enough either to hear or to see such miseries. (Josephus, J.W. 6.200–213)
Regarding OP’s question about who Eze 5:9–10 applies to, it should be noted that the context of Eze 4–5 shows that Yahweh’s words were directed at Israelites, and more specifically to the inhabitants of Judah and Jerusalem (v. Eze 4:6–7, 16; 5:5). Additionally, Le 26:29 and Dt 28:53 show that it was because of Israel’s disobedience to their covenant with Yahweh that he caused them to eat their children on the occasions mentioned above. Therefore, it is best to understand Eze 5:9–10 as applying specifically to members of the nation of Israel.
OP also asked about whether God could bring such a judgment as that described in Eze 5:9–10 on Christians who were to be a part of the falling away mentioned by Paul in 2 Th 2:3. Regarding this question, it is important that I first share my perspective on the time frame in which this event was to take place.
It is my understanding that the falling away of 2 Th 2:3 was an event that occurred within the lifetime of Paul and his contemporaries (i.e., during the first century AD). Evidence in favor of the notion that the falling away was occurring during that period can be found in the later New Testament letters, such as 1 John, in which the author writes:
Little children, these are the end times, and as you heard that the Antichrist is coming, even now many antichrists have arisen. By this we know that it is the final hour. They went out from us, but they didn’t belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us. (1 John 2:18–19, WEB; emphasis added)
As the author of 1 John makes clear, the believers of his generation were living at the time of the end and had already begun to see Christians depart from the faith. One of the primary causes for these departures was the influence of the deceivers and false prophets who had arisen and were leading people astray (1 Jn 2:26; 4:1; 2 Jn 7). Notably, Josephus wrote about several of these individuals who enticed the Jews to follow them and participate in their efforts to overthrow Roman rule in Judea.2 Some of these men, such as Simon bar-Giora, were even viewed as messianic figures.
Given that the aspirations and actions of these deceivers were more in line with the traditional views the Jews held about the Messiah, it should come as no surprise that Jewish Christians faced temptation to forsake Christ and join them in their nationalistic efforts. And undoubtedly, many Jewish Christians did give into that temptation (Mt 24:5 par., 11, 23–24), which means that they would have remained in Judea to fight the Romans, though Christ had previously warned them to flee when the Roman armies arrived (Lk 21:20–24 par.).
This detail is directly relevant to OP’s question because any of these Jews who turned from Christ and were in Jerusalem when it was besieged would have been subject to the same punishments as the rest of the Jews who were trapped in the city at that time.3 And of course, as I showed above, these punishments included parents being starved to the point that they ate their own children. Therefore, I do believe it is plausible that Yahweh may have punished some of the Jewish Christians who fell away from the faith by causing them to eat their children (or parents) during the siege.
1 Craig S. Keener, The IVP Bible Background Commentary: New Testament, Second Edition. (Downers Grove, IL: IVP Academic: An Imprint of InterVarsity Press, 2014), 108.
2 See Josephus, J.W. 2.433-434; 2.258-265; 2.652-654; 4.503-510; 6.285–288; Ant. 20.97-102; 20.167-172; 20:188.
3 Cf. He 13:14 where the author of Hebrews felt the need to remind his readers that they had “no enduring city” (i.e., Jerusalem) on earth and were to seek the one that was coming (i.e., the heavenly Jerusalem [He 12:22]).
Horsley, Richard A. “Messianic Movements in Judaism.” Edited by David Noel Freedman. The Anchor Yale Bible Dictionary. New York: Doubleday, 1992.
Keener, Craig S. The IVP Bible Background Commentary: New Testament. Second Edition. Downers Grove, IL: IVP Academic: An Imprint of InterVarsity Press, 2014.
Josephus, Flavius, and William Whiston. The Works of Josephus: Complete and Unabridged. Peabody: Hendrickson, 1987.