It’s difficult, because the word “All” would have had different connotations to Brahmins and the Buddha.
When the Buddha opposes Dependent Arising to these views, I’m sure by “all” he means the six senses and their experiences, similar to how he redefined “the world” (see SN35.23). To say that “all (continues to) exist” (i.e. to exist forever after death) is therefore eternalism; to say that “all does not (continue to) exist” is therefore the view of no life after death at all, which amounts to annihilationism. That’s how I interpret these statements at the moment.
The views of “all” being a unity or plurality to the Buddha I think are probably connected to the view that “one’s suffering is created by oneself” (i.e. unity) and the view that “one’s suffering is created by another” (i.e. plurality). These are said to amount to eternalism and annihilationism in SN12.17.
But clearly those are not really “cosmologies”. 
That’s because that is not what the Brahmins had in mind themselves. To them these statements were indeed cosmological.
I haven’t fully made up my mind yet what the exact Brahmanical ideas would have been—they probably changed over time, which makes it even more complicated… I plan to write on this in more detail soon (i.e. months from now). 
For now, a few quotes from Gonda’s paper Reflections on Sarva- in Vedic Texts:
The word sarvaṁ can very significantly be used in such a way as to suggest a sense opposite to illness and death. [The Upaniṣads say:] “He who (truly) sees does not see death, nor illness, nor any distress; he who (truly) sees sees the All (i.e., wholeness, completeness, integrity), he reaches (obtains) the All (wholeness etc.) in all respects (entirely).” In contradistinction to distress, illness and death, sarvaṁ must be a condition in which man is safe and uninjured; in which one has overcome death and reached “life eternal”.
It is therefore not surprising to read that by acquiring the insight into the essence of Brahman, or more briefly, by the knowledge of Brahman man becomes “All”: […] “Whoever knows ‘I am Brahman’, becomes this ‘All’.” […] And in the same Upaniṣad, the Self, the “Immortal”, the Brahman, the “All” are expressly identified: “This brilliant person which not being subject to death is in this space, and with reference to the individual, this brilliant person who is not subject to death and who is in the space in the heart, he is just this Self, this existence which is not subject to death, he is Brahman, he is Whole [sarva].” […] Brahman is what is the whole, complete here, is what is entire, perfect, with no part lacking, what is safe and well etc., i.e. Completeness, Totality, the All seen as the Whole. […] Sarvaṁ in this sense goes very well with the well-known “identifications” of Brahma with […] “the imperishable, immutable, unalterable” and with the conviction that Brahma is pure, free from evil, perfect, that it is truth, an indistinguishable unity, that “it transcends hunger and thirst, sorrow and delusion, old age and death”. The sarvaṁ is also “undecaying”.
Between the One and the many there is a relation of genetic dependence and existential contrast. In each of the many the transcendent unity is potentially latent, and by inversion of functionality it can be actualized as [knowledge] of the Whole, of the All. [… ] The Indians, identifying bráhman and sarvaṁ, choose this term for “intact, uninjured, entire, complete” to denote the Whole of Existence, the All, which in being eternally complete, is always free from decay, illness, and death. By realizing that he is sarva-, a man escapes death. […] The idea expressed by this word was […] that of “continuance of life”. The harbingers of death, all that which is injurious to health, hurtful, and prejudicial to the interests of earthly life was considered “harm, injury, loss, diminution, incompleteness”.
For Brahmins, this is no doubt partly behind “the all exists”, and it explains why it amounts to eternalism.
And the notion of All/Whole (sarvaṃ) is connected to the notion of unity as well, as Gonda explains. Other notions of unity and plurality are found in the Upanisads, such as:
“In the beginning this world was only brahman, and it knew only itself (atman), thinking: “I am brahman” As a result, it became the Whole. Among the gods, likewise, whosoever realized this, only they became the Whole. It was the same also among the seers and among humans. […] This is true even now. If a man knows “I am brahman” in this way, he becomes this whole world [i.e. unity]. Not even the gods are able to prevent it, for he becomes their very self (atman). So when a man venerates another deity, thinking, ‘He is one, and I am another,’ [i.e. plurality] he does not understand.”
We see here the Upanishads again posing a certain duality of views, one which the Buddha avoided by posing there is no self in the first place, whether identical with others or different.
