My initial impressions of what Bl. Hildegard presents from her visions in the
Book of Divine Works, versus the interpretation of Dr. Sugenis, shows me that I was correct in thinking that he is stretching her visions to fit his own cosmology.
I am reading the Robert Cunningham translation edited by Matthew Fox, so citations will correspond with that edition. The translation appears to use "circle" and "sphere" interchangeably in a similar sense to that of Aristotle.
Here, in summary, is what Hildegard says about the universe (Vision Two: 1):
1. The universe is like a wheel and is in the form of an egg. The "shape of the world" referenced by Hildegard constitutes the entire universe, not just earth.
2. The universe above the earth is composed of six circles
3. The first, a circle of luminous fire
4. The second, a circle of black fire, half the size of the first
5. The third, a circle of pure ether as large as the first and second circles put together
6. The fourth, a circle of watery air, which is the same size as that of the first circle
7. The fifth, a circle of sheer white clear air which "looked to be as tough as a sinew of the human body." It is the same size as the second circle and corresponds to the Firmament.
8. The sixth, a circle of thin stratum of air, which "seemed to raise up high" and is the residence of the clouds.
9. At the center of the wheel is a human figure, representing the dominion of Man in creation.
Further, Hildegard states that "All six circles were joined together without a wide space between them." At the circuмference of the wheel are various beastly heads; "a leopard, a wolf, a lion, and a bear." These exhale their breath into the pure ether, which permeates through what are the constellations of the Zodiac. Their breath is directed toward the earth, represented by the human figure.

The planets, contrary to Dr. Sugenis's interpretation of residing beneath the circles of luminous and black fire, the ether, and pure air (Firmament), in the expanse below; are, in fact, divided among the various circles of creation and reside within them. Three are in the luminous fire; one in the sphere of black fire; and the other three in the pure air below.
There are also sixteen major stars, which are set within the outer circuмference of the wheel between the four beasts (the four winds).
The stars themselves reside in the third circle of ether and fourth circle of pure air (Firmament), which corresponds to Genesis 1:14-15, 17 which places the stars in the Firmament, which, again is said in Scripture to be solid (Job 37:18) and Hildegard supports this in Vision Two: 6 and 7, where she speaks of the circle of pure air holding back the circle of watery air (waters above the Firmament): "The circle of watery air is meant to show that the waters flow above the firmament." (V2:6); "Under this circle of watery air is seen another circle of sheer white clear air, which has the same tensile strength as a sinew in the human body...this circle prevents flooding by the watery circle through its force and tension. For such flooding might otherwise submerge the Earth in a sudden and overwhelming precipitation." Not unlike Gen. 7:11.
The universe is said to be fixed in its shape, and is not set to change "and cannot be increased by anything new. It rather remains just as God has created it." (V2:2) As the universe is said to be like an egg, called "world-egg" (V2:3), it is a multi-layered structure not unlike a nesting doll and similar to the conceptions of the ancients. The world (i.e. universe, not just the Earth) is said to be "round and rotating". (V2:3)
The first circle is that of light, and is superior to that of the black fire, which is said to be "a judgment fire...produced for the punishment of evildoers." Most likely, as even Dr. Sugenis notes, it will be the same fire used to cleanse the world at the End of Time.
The third circle of pure ether is there to "restrain the upper and the lower zones so they should not exceed their proper dimensions." It further contains lightning "out of a flaming fire [those above] when the fire belches forth its flames." I speculate that this layer of ether above the Firmament and clear waters is where lightning phenomena, such as "elves", may originate. But that's beside the topic.
That's about as far as I've gotten so far in Bl. Hildegard's vision, as I just received the book today. But, what I've read so far does not correspond to what Dr. Sugenis makes it out to be, as something supporting modern cosmological assumptions.