1. Grammatically, it's possible to render Gen 1:1, as well as the syntactical relationship between 1:1, 1:2, and 1:3, in a way that indicates preexistent matter. But is that consistent with the aim of the text?
2. There's a sense in which the original audience for Genesis weren't orthodox Jews. Rather, the Pentateuch is what made them orthodox Jews. It has a pedagogical or catechetical function in teaching them how to think properly about God and their place in the cosmos. Imagine what they believed before they had the Pentateuch. What they believed when they read it or heard it for the very first time.
I daresay their beliefs were a hodgepodge of folklore, local mythology, perhaps some oral traditions about Abraham, and their memories of the Exodus. To some degree, their default frame of reference is pagan mythology and primitive folklore.
One purpose of Genesis is to set the record straight. To teach them what really happened. To correct the heathen creation myths floating around the ancient Near East. Not necessarily by directly alluding to them, but by presenting the real history of events.
3. When theologically orthodox scholars and commentators leave the door open for preexistent matter, I think they have in mind a three-stage process:
i) In the beginning was God
God did not come into being. He always existed.
ii) Preexistent matter
At some point, he made matter. This was the raw material for creation.
Gen 1 picks up where (ii) leaves off. God organizes the preexistent matter into the universe. Preexisting matter is like the clay from which God fashions a pot.
But a basic problem with this analysis is that, from a pagan perspective, there's no presumption that God or gods preexist nature. Indeed, the presumption is that nature or the world process preexists God or gods. Nature never came into being. Gods came into being. Gods are the byproduct of the world process. So they'd understand the three-stage process this way:
i) In the beginning matter (or nature, or the world process)
ii) Then God
iii) Then God fashions preexistent matter into something more specific
If Gen 1 doesn't rule out preexistent matter, then it doesn't rule out paganism. But that would be counterproductive to the narrator's aim.
4. If possible, it gets even worse. Modern western readers think of natural elements as inanimate or impersonal. But in paganism and animism, it isn't just preexistent "matter" or stuff. Rather, darkness might be a god, the deep might be a god. Indeed, the original gods, from whom Yahweh came.
If Gen 1 doesn't rule out preexistent matter, then it doesn't rule out polytheism or cosmogony. Once again, that would be counterproductive to a major aim of the narrator. For those reasons alone, I think Gen 1 must intend to convey creation ex nihilo.
5. But here's another consideration. If the narrator wanted to convey creation ex nihilo, how could he do so with the available vocabulary and categories? One strategy would be to express the idea through negations. Indicate that before God's creative activity, there was nothing apart from God.
Look at v2. Darkness is a negation. The absence of light. And darkness is more abstract than night.
To be formless is to have no structure. And a void is synonymous emptiness. Vacuity. A blank.
Taken in combination, isn't this a way of suggesting that prior to God's creative activity, there was nothing at all? An "earth" without form and void, covered in darkness, is a paradox. A way of saying there was no earth. For the "earth" in v2 is defined by totalistic negations.