Perhaps: “seed” is singular at Gen 3:15 (זַרְעָ֑הּ, zarah), and (for reasons given above) does refer to one particular man. But the grammatically singular form of the word does not clinch the question, because there are other cases later on where it is used in prophecy in the singular and where it definitely serves as a collective noun (like “one herd” or “one people”). So e.g. at Gen 12:7 we have “Unto thy seed will I give this land.” The word in this place is nearly identical, singular ( לְזַ֨רְעֲךָ֔, zaraka), but here is widely understood to mean “Unto thy descendants (pl.),” i.e., the children of Israel. And this promise was fulfilled: Abram’s seed (descendants, the children of Israel) did come to dwell in the land. But this same later promise to Abram will also be fulfilled by Christ in the Millennium. That means the ambiguity of grammar (between a singular noun used in a singular sense and in a collective sense), both in Hebrew and in English (“seed” also serves as both a singular and a collective noun), permits a doubly-fulfilled prophecy. All that said, perhaps it is best to regard the protevangelium, too, as to be doubly-fulfilled, by God’s people indeed but most outstandingly by Jesus of Nazareth. The saints will crush the serpent’s head; but first and foremost Jesus crushed it in his resurrection and will finally crush it at the end of days.