There are a couple of things worth discussing here.

You rightly note that the narrator writes “looking back”, as it were, from a Christian context – and it’s also worth bearing in mind that the only surviving manuscript was produced and probably read in a monastic context in the firmly Christian late-tenth century. The first lines you quote, in which the Scyldings turn to idolatry to protect themselves from Grendel and are condemned by the narrator in pretty damning terms, is probably the most strikingly Christian passage in the whole poem. It is also a very interesting passage, and generations of critics have contested its precise meaning and structural role in the poem.

I can’t claim to wield any final authority on Beowulf, but there are some specific phrases that have a bearing on this question. Clearly the narrator is talking about the Christian God; the constellation of names makes that fairly clear. A slightly more literal translation than Alexander’s brings that out nicely. If you’re interested you can view the original text here; the relevant passage is on ff. 135v-136r. The narrator tells us that the Scyldings ne cuþon (did not know): metod (Providence, literally “the one who metes-out”), dæda demend (the Judge – ie “the Deemer” – of Deeds), drihten god (Lord God – “Drihten” is also used for mortal lords), heofena Helm (the Protector – same root as “helmet” – of the Heavens) and wuldres waldend (Wielder of Glory).

So far so straightforward. But who exactly are the Scyldings worshipping instead? We are told only that they put their hope in helle, and in the gastbona who dwells there. Gast-bona is (as far as I know?) not a compound attested outside of Beowulf, but is usually translated as something like “spirit-slayer” – its constituent parts are cognates of the modern words ghost and bane. I note that the early scholar George Stephens argued in 1878 (reprinted in T. A. Shippey and Andreas Haarder, eds, Beowulf: The Critical Heritage, 1998, pp. 387–9) that this was philologically nonsensical and that the word should be translated as something like “giant-slayer”, but this interpretation never seems to have gained much traction. Regardless, the meaning is ambiguous. Are the Sycldings worshipping the Christian Devil, or a pagan God (Odin or similar) who the narrator considers a devil, or a pagan devil to whom they have turned in desperation in the face of wræc (misery, as in “wrack and ruin”) and modes brecþa (heartbreak)? Pleasingly, it is unclear whether said misery and heartbreak describes the devastation caused by Grendel or the damnation caused by heathen worship.

The most important phrases with regards to this question, at least to my mind, are a little more prosaic: monig oft (many often...) and the various uses of “they” that follow it – Hwilum hie (Sometimes they...), metod hie ne cuþon (they did not know Providence/God). We are dealing with many of the denizens of Heorot engaging in heathen practices – be they heathen by Christian standards or those of their own religion – but not necessarily all. The hie of the passage is not necessarily every Scylding and most importantly it doesn’t seem to include Hrothgar, who is mentioned at the top and tail of this passage as made miserable by this turn of events. Also, of course, it cannot include Beowulf himself, both because he is a Geat and because he has yet to arrive in Heorot at this point in the story.

This is important because Beowulf, like plenty of other Anglo-Saxon literature that is simultaneously Christian and Germanic-heroic, leans pretty heavily on the “noble pagan” topos. For all that Alcuin of York had cause to remark in 797 “What has Ingeld to with Christ? The house is narrow; it cannot hold them both”, it is telling that all four of the so-called Anglo-Saxon poetic codices contain poems which blend strands of both cultures. Beowulf and Hrothgar may not be Christians, but nor are they grouped in with the damned idol worshipers, and their words and actions fit into a worldview that is sort of “morally”, if not strictly spiritually, Christian.

This is probably the best way to make sense of the other passages that you quoted (as well as the various usages of names for the Christian God used by pre-Christian characters in the poem). The second passage recounts the singing of the scop in Heorot which brings about Grendel’s ire. The poet sings about Creation, and not in terms that are especially Christian – for what it’s worth, the word Alexander translates as “Lord” is ælmihtiga (Almighty). There is the possibility, of course, that we are meant to understand that the denizens of Heorot are Christians and only lapse later, but this need not be true. The song is simply a creation story that might fit any monotheistic religion (or a polytheistic religion with an almighty head god). Whether or not the Beowulf poet actually had any detailed idea of the nature of pre-Christian Scandinavian religion is difficult to say, although scattered references to figures like Odin and Weland the Smith in Anglo-Saxon literature (as well as on the wonderfully enigmatic Franks Casket) suggest that this was possible. Regardless, all the really Christian stuff here comes a few lines down, and it comes from the narrator: that this song also fits the Christian Creation myth, and that Grendel is caines cynne, and that he has suffered at the hands of the ece drihten (eternal Lord), the metod. The song may or may not be Christian, but its theology is Christian-like enough to use the word ælmightiga for some all-powerful divine force.

The case is similar in many other religious moments in the poem; when Beowulf and his men arrive by boat, they gode þancedon (line 227, “thanked God”); Beowulf says of his battle that he would have died nymðe mec God scylde (line 1658, “had God not shielded me”); when Beowulf leaves to fight the dragon, Wiglaf says that God wat (line 2650, “God knows") that he would rather die by Beowulf’s side. All of these references have the same flavour; they could be said by a Christian of the Christian God, but they need not be. In their battles with devil-ish monsters and their faith to a Christian-like God, Beowulf and Wiglaf can function as Germanic-heroic heroes in a Christian text. It’s worth paying that sort of scrutiny to all references to God in the poem – considering whether the narrator or a character is speaking, and how explicitly Christian or not the reference might be, can be very helpful in sorting them all out.

Hrothgar, your third example, is something of a special case. His references to God, and in particular his “Sermon” (line 1698 onward) are of an especially densely theological nature, and are not as easily unloaded onto Odin or similar. I don’t want to overstate things here, as the nature of Hrothgar’s faith in the poem is another critical hot topic – and as someone who doesn’t really specialize in Anglo-Saxon literature I’m wary of oversimplifying. Nevertheless, even Hrothgar makes no reference to Christ, or any Biblical event. His God is an almighty (either monotheistic or head of a polytheistic group) Providence, a being that rules over all. Some of the Old English terms for the Christian God are particularly favourable to this kind of interpretation: the Meter and the Wielder, as mentioned above, but also the scyppend (Shaper). But Hrothgar’s God doesn’t have to be the Christian God; we are again looking at something closer to a “noble pagan” moral and theological similitude than an outright spiritual identity.

I hope that goes some way to answering the question, or at least showing one of the standard ways that critics and literary historians have dealt with the open issue of religion in Beowulf as well as Anglo-Saxon heroic literature more generally. Naturally there’s a great deal of material out there on this stuff, but I’ve found the following particularly useful:

Patrick Wormald, ‘Bede, Beowulf and the Conversion of the Anglo-Saxon Aristocracy’, Bede and Anglo-Saxon England, ed. Robert T. Farrell (1978), 32–90. A classic. Also reprinted in Wormald’s The Times of Bede (2006).

Carol Braun Pasternack, ‘Post-Structuralist Theories’, Reading Old English Texts, ed. Katherine O’Brien O’Keefe (1997), 170–91. This book contains many useful essays, several touching on Beowulf. I single out Pasternack’s because of her excellent discussion of the idolatry passage, and its effect on the poem’s religious tension.

Andy Orchard, A Critical Companion to Beowulf (2003). The ‘Religion and Learning’ chapter (130–68) may be particularly useful here. If there is any such thing as an authority on Beowulf it is probably Andy Orchard. All of his work on the poem (and much else besides) merits a look.