The idea of the necessity of knowing what Dassein is in order to understand what beingness is is never explained, it is just deliberately stated as fact. And also how he rejects the ontological ideas surrounding “subject” and “object” saying they come from the formalization of language and there's something before that that is more attuned with the truth without explaining why he believes it is so. Is there something in his philosophical framework that I am missing, or are his ideas never really justified through ordinary logical reasoning?
It's no unwarranted exaggeration to say that Heidegger assumes familiarity with the phenomenological tradition, and in particular with Husserl. But the bit of Husserlian doctrine most crucial to the question you pose might not be what you expect. In particular, it would be natural to suppose one must study up on Husserl's (in)famous phenomenological method and his doctrines of intentionality and of the transcendental ego, etc. But Heidegger rejects Husserl's phenomenological method and does not take over and refine any of his phenomenological results, with the debatable exception of certain of Husserl's notions about temporality. There is certainly some illumination to be gained from understanding Husserlian phenomenology vis-a-vis Heidegger, but Heidegger's own phenomenological methods and results can also stand on their own (I would contrast this with Merleau-Ponty, who I see as taking over and amplifying many of Husserl's actual phenomenological results).
Rather, it is an Husserlian innovation in ontology that is crucial for interpreting Heidegger's conception of fundamental ontology. Husserl has a categorical ontology but he distinguishes between formal and material ontology. Material ontology places particular objects under a hierarchy of generality. So, for example, a particular goldfish is placed under the species 'fish', which is a species of 'animal,', which is in turn a species of 'natural object', which is a species of 'spatial object', and so on into higher and higher generality. By contrast, formal ontology abstracts from concrete particulars to their bare "this-ness", their being something here before me; as Husserl puts it, the "empty something" or the "determinable X". Categories and relations of formal ontology apply to any thing as something whatsoever ---that is, as a being. Hence, they apply to all beings, no matter their material ontology. There is thus a sense in which formal ontology is more fundamental than material ontology.
Heidegger picks up this thread and claims that phenomenology is fundamental ontology. What he means is that phenomenology clarifies the sense in which something is a this-here. This-here is a purely indexical notion, which makes essential reference to the lived experience: something is only this relative to some disclosing function as lived by Dasein. So until we have understood this essential, inner relation between Dasein and fundamental ontology, it is not that we cannot understand Being but that we cannot even properly pose the question. Thus Heidegger interprets formal ontology---which becomes fundamental ontology for Heidegger--as an approach to lived experience. While Aristotle had tried to grasp the being of beings, he had done so only by trying to understand what was common to every category of a material ontology. He therefore missed the true problem of Being. Heidegger claims that understanding the Being of beings requires understanding their disclosedness as a something whatosoever, which in turn is a structure that depends on the categories of Dasein's lived experience.
In short, I would submit that Heidegger's notion of fundamental ontology assumes familiarity with Husserlian logic, but he does not spend any time getting his reader up to speed on this fact.