The way I see it Heidegger took a phenomenological look at language, where to Husserl he drove phenomenology towards a way of uncovering the foundation of science.
Phenomenology for Heidegger may well be ontological, but phenomenology is not ontological it is phenomenological. Dasein as I see it is an attempt to establish a non-existent point of presence outside of presence. There is no need for dasein in phenomenological investigation because it merely covers up phenomenology more than it is already covered up in being called "phenomenology".
Heideggers focus is language not phenomenology. He partakes in a phenomenological investigation of language.
As an anagolous view of what "dasein" means I can say that it is "being-there" as the instant now looking upon itself being. Heidegger differs from Husserl, in my understanding, in that he vjews existence as grasping towards our own sense of being where Husserl makes no such assumption as to what is being grasped at only that there is a grasping that may uncover our sense of self or the sense of the universe. Such a difference is of no significance directly in the phenomenological procedure. Although, that said, Husserl was clear as to his intent and how he was grasping at the founding of science in order to give science a new founding, ironically by further sedimenting it during the procedure of uncovering its origin.
What I have noticed with Husserl is that people often mistake his use of "object" as meaning something "in-itself" or out-there. All that matters in phenomenology is constitution of the phonemonological and givenness of its object (object may well be dasein, dog or the number one). The idea of dualism has no place in phenomenology other than as an "object" of phenomenological investigation which is seen through application of logic and through which Heidegger holds fast above and beyond within his created object that lies outside phenomenological experience.
My question only remains as to what use "dasein" is as a concept to me and whether I wish to use it. Of course I may very well never be able to grasp what the concept is and so fail Heidegger as much as he has failed to explain to me what it is he is saying. I know new concepts are intially hard to hold once you're newly introduced to them. I think for this reason he hides in an excuse of hermeneutics and covers up the meaning of the concept with its none meaning in order to give a pretense of meaning, not just to me but also to himself.
He starts with semantics and moves into etymology and produces hermeneutics. He does admit to our lack of grammatical structure to deal with what he says yet soldiers on regardless further burying the phenomenological procedure behind him.
I cannot remember if he mentions "I"? I have being. I am a being. I am being. Without language there is no "I" only being. It is sophistic to say something like being of being prior to a conceptually verbal "I".
Dasein is the pre-ontological understanding of being that lacks full accessibility ontologically?
What is ontic-ontological priority of Dasein? What is its "genuine ontological structure"? Does this really mean anything or is he hiding behind meaning with the excuse of interpretations? Basically he is being sophistical in his approach and is not inclined to show any reason for doing such a thing and leaves the reader hanging on accept in several cases where he takes, often several, pages to say some that is summed up quite simply and logcially in the final paragraph. This is done a number of times and leads the reader to believe in some sense when there is none in other areas (or I am just not smart enough to understand or lack the ability to find direct value in his words).
One thing I can say. I often make less sense than Heidegger!
Note: I am always very scathing in me view of philosophers. I am equally scathing towards Husserl. It is the best way I know to find sense or meaning.