“This is the direction in which the late Heidegger seems to move, if still insufficiently, with the idea of a final event or appropriation (Ereignis) in which what is appropriated is Being itself, that is, the principle that had until then determined beings in different epochs and historical figures. This means that with the Ereignis (as with the Hegelian Absolute in Kojève’s reading), the 'history of Being comes to an end' (Heidegger Zur Sache des Denkens p. 44), and the relation between Being and being consequently finds its 'absolution'. This is why Heidegger can write that with the Ereignis he is trying to think 'Being without regard to the being,' which amounts to nothing less than attempting to think the ontological difference no longer as a relation, and Being and being beyond every form of a connection.”
It's right to point out the risk of relapsing into old metaphysical habits of thought. But there's no danger of this if you keep your wits about you. I don't find anything about "absolution" on p.44 of Zur Sache des Denkens, but there is mention there of the history of being coming to an end. This must be so once the step back from being thought as the beingness of beings is taken to the giving of being itself. The step back is a matter of learning to see something new that metaphysics was unable to see explicitly. It did not see the intimate connection between being and time, and had recourse instead to an _idea tou agathou_, a summum ens or the like conceived as Ur-Sache, i.e. as primal thing. Hence the fatal ambiguity of Metaphysics from the start as ontology (the investigation of _to on haei on_), on the one hand, and as _epistaemae theologikae_ (theological knowledge), on the other.
In any case, Agamben muddies the waters by characterizing Ereignis as a "final event or appropriation (Ereignis) in which what is appropriated is Being itself". The propriation that eventuates is that of being itself and human being to each other. Since being itself is Anwesen (presencing), which unfolds as the three-dimensional time-clearing of the Da, it's equivalent to say that the Da and Dasein are enpropriated to each other; they belong to and need each other. This is the retrieval (Wieder-Holung) and re-sending of Parmenides' message: "For minding and presencing are the same." (Frag. III) (Cf. my 'Out of your mind? Parmenides' message'.)
Ereignis is the abyssal (groundless), hidden (_laethae_) It that gives. Its giving is twofold. Firstly it "reaches" the three-dimensional interplay of the ecstasies of time to each other through to human being itself. The Da as the interplay of presencing and absencing impacts, affects human beings, rendering them as Dasein. Secondly, It gives the respective hermeneutic As of each historical epoch which defines AS what or who presents present themselves in the Da (time-clearing).
Is Heidegger "attempting to think the ontological difference no longer as a relation, and Being and being beyond every form of a connection," as Agamben claims? No. Why not? Because the ontological difference is now thought as the sending of the historical casts (the hermeneutic As in the plural) of the presencing of presents, and hence still as a "relation", but seen differently. This sending can only eventuate within the open time-clearing of the Da to which Dasein belongs and which in turn needs Dasein.
Agamben is off on a wild goose chase insofar as he wants to 'explain' the thinking of Ereignis from the ontic political situation of the 1930s. Such explanations are worth nothing for issues of phenomenological thinking which is grappling with those simplest thoughts through which history is shaped decisively, definitively. Phenomenological thinking is thus a 're-vising', a 're-seeing' of these sendings of historical casts, a learning to see what we already see unthinkingly, in order to re-cast a more appropriate hermeneutic As into the future that may arrive one day. Any attempt to 'explain' phenomenological thinking politically is misguided, since the political itself as an historical way of worldsharing is itself a phenomenological-hermeneutic question. Therefore, you have to enter this hermeneutical circle to gain clarity about the political. In the West, this question concerns the historical issue of (the possibility of) FREEDOM in worldsharing.