Old Testament verses other than Genesis 3:15 speak of the "seed" of a woman. How, then, does it indicate a virgin birth?

Seed, like at Gen. 3:15 or 16:10, etc in Hebrew is a masculine noun. In our typical human knowledge, the implication is the male has the seed and the woman brings forth children; hence the idea that the seed simply means descendants in a normal way, even though it speaks of her seed.

The uniqueness of the word as given to the woman at Gen. 3:16 is clarified first with Isaiah's 7:14 and amplified at Paul's Gal. 3:15.

Isaiah prophecies of a virgin (or young woman) giving birth. It's no sign for a woman per se to give birth. It was and had to be a virgin to qualify as a sign from God. Yet, we know seed is the masculine noun. Again, the implication of it is from a man, but no, it is a virgin! Huh? How?

Paul explains it that the seed of Gen. 3:15 is singular, referring to one Christ Jesus. It is not her descendants, her seed, but still the masculine noun seed.

So, we have God saying to the woman her seed will bruise the serpent, but there is no masculine participant for the singular seed that Paul details, rather it is the virgin who brought forth, and we know that "overshadowing" of the Holy Spirit seed as Christ Jesus.

http://biblehub.com/hebrew/2233.htm

https://en.wikibooks.org/wiki/Hebrew/Nouns

Women and their seed is merely a reference to the women's offspring, children, and/or posterity. ¹

Gen 16:10 is in reference to Sarah's descendants being unnumerable

Gen 24:60 Rebekah's descendants will possess the gate of those that hate them

Gen 4:25 talks about Seth being another son since Cain slew Abel

Lev 22:13 mentions a priest's widowed daughter without a child

1 Sam 2:20 Elkanah and his wife will be blessed with children as they have prayed them

Gen 3:15 is about enmity between Eve's descendants and Satan's descendants/followers

None of these prophecies explicitly refer to the virgin birth. If it is your understanding that it does, none of the other references agree or disagree, as they are about different people.

_{1 http://biblehub.com/hebrew/2233.htm}

There are a number of different aspects intertwined in your question and I cannot answer all of those aspects together, but I hope that by making one point clear that it will make others clear also.

Green’s Interlinear Hebrew and Young’s Literal translation agree on the English translation of the Masoretic text of Genesis 3:15 (Young always uses a present tense as a literality) :

And enmity will I set/I set between you and the woman, and between your seed and her seed He will/doth bruise you the head. And you shall/dost bruise him the heel.

‘Seed’ here is singular. Paul points out that ‘seed’ can be singular in Galatians 3:16 :

Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.

So also in Genesis is it said by God to the Serpent of the future seed of the woman :

He (singular) shall bruise thy head. And thou shalt bruise his heel.

Here, a rearrangement is prophesied by God. For mankind had fallen under the Serpent by transgression. The Serpent had gained control by deceiving the woman, who in turn caused the man to transgress and they both had fallen under the power of the Serpent. Cast out of Eden they were to till the cursed ground until they died and were buried in it.

But in the promised rearrangement, the seed (singular) would be above the Serpent. From above, he would bruise the head. Retaliating, the Serpent would only be able to bruise the heel of the one above him.

Then how the rearrangement ?

In subsequent prophecies, more would be revealed. But for the time being that was all that was revealed to Adam and to Eve (and to the Serpent). But it was enough for Adam to name his wife - Eve, in recognition that she would be the mother of all living. That is, all who truly live.

She would also be the mother of all the seed of Adam, but that is a different matter.

From Eve there was promised, by God himself, a seed (singular) through whom would be a rearrangement that would overcome all that had transpired in Eden. That which the Serpent had done, that which the woman had done and that which Adam had done - all would be resolved by the coming seed.

Just how that seed would come - and just how that rearrangement would be accomplished - are made clearer in subsequent revelations from God. It was not yet revealed how a seed would come who would not be subject to the consequences of Eden. How could it be possible ? was a question that could not - then - be answered.

I hope that this clarifies just one aspect of the research in which you are engaged.