Heiser argues for the same position as Paul Sanders in The Provenance of Deuteronomy 32 (Brill, 1996), and both make good arguments about the form of the infinitive, the object relation of גוים (are the nations given as an inheritance or are they the ones who inherit), the function of the adversative marker, as well as the contextual links of the passage to the rest of Deuteronomy. But missing in both is a discussion of what to me makes a subordinationist reading really possible: the relation of v. 8b to 9. In the second verse, we read that Israel is Yahweh’s portion (חלק יהוה) and his inheritance (נחלתו). In v. 8a, we read that Elyon caused X to inherit (בהנחל) the nations (accepting Heiser’s analysis of the object relation as accusativus rei). Then in v. 8b we find that X corresponds to the “sons of God” who are the ones who inherit the nations. For Yahweh to have a share (חלק) in this inheritance, this makes Yahweh have the same semantic relationship with the hiphil verb as the sons of God. The non-subordinationist reading identifies Yahweh with Elyon and essentially makes Yahweh divide up the nations for the lesser divine beings but he takes Israel for himself. But 8b seems to militate against this. The boundaries of the peoples (גבלת עמים) were fixed by the number of the sons of God. Since each nation has a territory, this fixes the number of nations according to the number of the sons of God (thus each divine being gets to inherit a nation). There is no indication that Israel is an exception to the rule; it is indeed one of the nations given as an inheritance as v. 9 states, so its territory was determined by the number of the sons of God. If Israel was one of the nations given as an inheritance to the sons of God, it is hard not to read this as implying that Yahweh was among the number of the sons of God.
So the internal features of this poetic unit seem to clash with the application made of it in Deuteronomy. The author(s) of Deuteronomy, and certainly the Song of Moses itself, adopted the same non-subordinationist understanding argued by Heiser that Yahweh was the one who apportioned out the nations. The same is true with respect to Jeremiah 10:16, which is the earliest allusion to Deuteronomy 32:8-9 outside of D. According to Mark S. Smith, who cites Deuteronomy 32:8-9 as evidence of an earlier stage of Yahwism prior to Yahweh’s identification with El, the composer of the Song of Moses “understood Elyon as a title of Yahweh” but the text itself contains a “concealed inconsistency” vestigal of an older theological scheme (“What is a Scriptural Text in the Second Temple Period,” in The Dead Sea Scrolls at 60, p. 285; Brill, 2010). In other words, the author of the poem may have given this unit a secondary application that produces dissonance in the text. I think it is worth considering that v. 8-9 is a quote or fragment from an older poem or psalm that otherwise perished. Eckart Otto (“Singing Moses: His Farewell Song in Deuteronomy 32”, in Psalmody and Poetry in Old Testament Ethics; Bloomsbury, 2012) finds that the poem is not older than the narrative frame it is placed in, and observes that it is highly allusive of other known psalms such as Psalm 74 (74:2, 4, 18 = v. 5-6, 74:18 = v. 21, 74:19 = v. 24), 78 (78:8 = v. 5), 90 (90:13 = v. 26), 91 (91:4 = v. 11, 91:5-6 = v. 24), and 92 (92:16 = v. 4). Note that these psalms are clustered close together in the Psalter. So it is quite possible that v. 8-9 incorporates language from another psalm no longer extant. Indeed there is a particular parallel in Psalm 74 (74:17 = v. 8), but only the phrase הצבת כל גבולות.
Another possibility that just occurred to me is that v. 8-9 presumes the myth behind Psalm 82, which I agree posits Yahweh as at the head of the divine council. So what we have in the transition between v. 8b and 9 may be a change of situation. Elyon gives the nations as an inheritance to the sons of God. Then the judgment presumed in Psalm 82 occurs (if this is a theomachy then the gods who are judged and die are analogous to the olden gods in the ANE relocated to the underworld). Yahweh finds that the gods of the nations of Canaan were guilty and condemns them to die. He then in v. 9 inherits a new nation (not one of those apportioned out earlier by Elyon) via the Sinai covenant and takes the land that the gods of Canaan had previously governed. The problem with this might be that כי in v. 9 may not bear the brunt of signalling a such a significant contrast (which would introduce a second instance of inheritance-giving). I’m not sure if this means that Yahweh was necessarily the same as Elyon originally.
With Yahweh elsewhere described as coming from the south, from Sinai and Paran (Deuteronomy 33:2; cf. the desert locale in 32:10), it seems that within this mythological tradition, he may have already possessed a territory and people from the prior apportioning of the nations (receiving an inheritance from Elyon). Another interesting feature of Deuteronomy 33:2 is that Yahweh seems to be accompanied by all the sons of God (holy ones) as his divine retinue (cf. 32:43 LXX with the gods bowing to Yahweh). This accords also with the role of Yahweh in Psalm 82, with all the other gods subordinate to him. I wonder if the change in situation also involves the sons of God giving Yahweh universal authority over the world. This would then mean that the nations no longer sacrifice to the gods previously apportioned to them, but to demons and “no gods” (v. 17). If the other gods originally had nations to rule over (via the giving of the nations as an inheritance) but this is no longer the case, and if Yahweh’s possession of Israel was acquired and not part of the original apportioning of humankind, then this leaves the possibility that Yahweh was not always the supreme sovereign but underwent a growth in authority as Baal did in the Ugaritic Baal cycle (in which Baal was the young dynamic god seeking kingship and El was the passive aged god over the pantheon).
In From Epic to Canon: History and Literature in Ancient Israel (Johns Hopkins, 1998), Frank Moore Cross made the claim that young gods may adopt the names of olden gods, mentioning Olam and Elyon as two examples of overlap in the names of distinct gods (pp. 77-78). El himself was originally distinct from Elyon (as the Sefire treaty shows) and Philo of Byblos portrayed Elyon as the grandfather of El, with Ouranos (Heaven) as an intermediate generation, from whom El wrested power as the young dynamic god; this theogonic scheme matches the sequence in Hittite/Hurrian texts consisting of Alalu < Anu < Kumarbi < Teshub (= Elyon < Ouranos < El < Zeus-Demarous (Hadad), see Marvin Pope’s El in the Ugaritic Texts (Brill, 1955), p. 56. Interestingly, Heaven is named as a witness in Hittite treaties, often with Earth, and arṣ w šmm “earth and heaven” appears as witnesses to treaties at Ugarit as well, and they appear as olden gods on pantheon lists (KTU 1.47, 118). This closely parallels what is found in Deuteronomy, where “heaven and earth” are named as passive witnesses in the divine lawsuit (4:26, 30:19, 31:28, 32:1). So if this element of West Semitic polytheism was drawn upon in Deuteronomy, it is plausible that an older tradition on Elyon was also incorporated into a Deuteronomistic poem expressing Yahwistic monotheism.