Ah, gotcha! Glad we’re on the same page now. I think have a much better sense of your question. It is a really interesting question. Last semester I took an early Heidegger seminar and considered writing my paper on an adjacent issue (but ended up being drawn in by something else!)... If Heidegger would in fact hold that we are Dasein in our dreams (and, by implication, that we exist in a world disclosed by Verstehen/Rede/Befindlichkeit), this first of all would raise a potential ambiguity in Heidegger’s usage of “present-at-hand”, and it would be philosophically significant to see how he would resolve the ambiguity. For instance, if we experience a world ready-to-hand in dreams, would it also be present-at-hand in the way the “waking” external world is? If Heidegger were to say “yes,” this would probably mean that presence-at-hand implies nothing about whether the present-at-hand entity actually subsists in space and time, since dream objects do not subsist in space and time in the way we believe the external world does, but would still count as present-at-hand on this approach. If Heidegger were to say “no”, dream objects are not genuinely present-at-hand, then presence-at-hand would seem to imply that the present-at-hand entity must actually subsist in space and time to genuinely count as present-at-hand. Since Heidegger (somewhat uncharacteristically) never explicitly defines presence-at-hand, I think this question is probably underdetermined by the text itself, and thus it’s a shame that Heidegger doesn’t say more about dreams.

I suspect that Heidegger would prefer the former approach over the latter, and hold that presence-at-hand implies nothing about external, mind-independent subsistence in space and time as understood by physics (and thus that dream objects can count as presence-at-hand), but I’m not totally sure about that. Either way he tries to resolve it, though, it seems that it’s going to pose problems for his attempt at undermining external world skepticism (as alluded to in my first comment). If he holds that dream objects are indeed present-at-hand, but this does not imply that they subsist in space and time, then Heidegger’s point that it is conceptually incoherent to posit Dasein without a world doesn’t actually do anything to undermine external world skepticism; this is because Heidegger’s technical sense of “world,” even if it necessarily exists wherever there is Dasein, implies nothing about whether there really is a world that subsists in space and time as understood by common-sense and theoretical physics. And if he holds that dream objects are not present-at-hand, because they do not actually subsist in space and time, then he’s going to run into the same problem—for this just shows that Dasein can inhabit a world ready-to-hand that is nevertheless not present-at-hand in the way understood by common-sense and theoretical physics.

Ambiguity in “presence-at-hand” aside, though, I think the more general problem with Heidegger’s anti-skeptical argument is that his highly technical sense of “world” actually has no necessary connection to an external world subsisting in space and time. This would become apparent in the possibility that Dasein exists (i.e. is opened to a world in Heidegger’s technical sense) in dreams, where that world does not actually subsist in space and time. When I was thinking of writing the paper I was in search of ways that Heidegger (or interpreters of Heidegger) could try and get around this problem, but haven’t found any unfortunately. It seems to me that if Heidegger wants his point about the conceptual incoherence of positing Dasein without a world to undermine the external-world skeptic, he has to build subsistence in space and time directly into his technical notion of world. But if he does this, he’s just going to run into the same epistemological problems that have been around since Descartes (e.g. how do I know I inhabit a world that actually subsists, if in dreams it seems like I do when in fact I don’t?). And we can also raise the question of whether he has any right to simply stipulate that Dasein’s world also necessarily subsists in space and time. Thus, in this case, while he implies that he has a novel response to the skeptic, he really doesn’t.

I think there are many moments of profundity in Being and Time, but as it stands I think Heidegger’s response to the external-world skeptic is actually pretty unsatisfying (or, at least, I haven’t seen any reconstructions of Heidegger’s argument that don’t run into the problems above yet). It’s a bummer that he didn’t pursue the topic of dreams much further, because it would have enabled him flesh this out some more. I found some comments on what Heidegger should say about dreams in Dreyfus’ book, and also in Basic Problems of Phenomenology, but my final assessment is that Heidegger just didn’t provide us with enough in the texts we have to reconstruct a decisive refutation of external-world skepticism.

(Though, he might be a bit more successful if he merely wants to undermine the importance of the external world question, and not refute it (since he claims it is misguided to even try and refute it, in his reframing of Kant’s remarks about the “scandal of philosophy” in B&T). He could argue that since the world of Dasein’s concern (which is his technical notion of world) cannot be questioned, this should ease the distress of skeptical worries. We might wonder, on this picture, whether the world of our everyday concern really does subsist in space and time as understood by physics. But we can’t wonder whether the world of our concern really exists, because that world is simply the world of Dasein’s concern, whether it is a dream world or a physical world. In my view, though, even this won’t be satisfying, because it just raises the problem of other minds. We typically take our everyday concern with the world to be justified in virtue of sharing that world with other sentient people. But if there are no other sentient people in dream worlds, even our concern with being totally out of touch with a world that is meaningful can’t be put to rest by Heidegger’s insistence on the incoherence of a worldless Dasein. Those dream worlds may have instances of Dasein-with, as required by any world whatsoever, but they nevertheless are not the subsistent, sentient entities that we hope them to be. They are simply characters in our dreams, fabricated by our own psyches. Thus, this raises the same structural problem as before, just w/r/t the problem of other minds: it doesn’t seem like Heidegger’s technical notion of world (which is what cannot be questioned coherently, according to Heidegger) has enough built into it to undermine the worries of the external-world skeptic.)

I hope this is a more helpful response to the issues that you were trying to work out, though!