No Blunder at All
The word "seed," whether Hebrew or English, is often used in a figurative sense to refer to one descending from another (and not normally to the actual sperm or egg of the parent that is the source of propagation). The word can have a singular or a collective meaning. Even a collective meaning, however, is viewing the individual elements as a unit together, as a whole. So even the plurality contained within the collective is considered as a single unit.
Thus, to the contrary, it clearly does not have a plural meaning, at least not apart from the collective. The Hebrew could have used a plural form (it is not that the language was incapable of forming a plural of the word), but God does not so move the writers of the Old Testament.
This singular as collective is Paul's emphasis in the distinction. Now let's reexamine Genesis 13:15-16 (using the KJV, as you did... other translations over translate this by putting "descendents" or "offspring," both of which lose the collective idea that the Hebrew carries).
15 For all the land which thou seest, to thee will I give it, and to
thy seed for ever. 16 And I will make thy seed as the dust of the
earth: so that if a man can number the dust of the earth, then shall
thy seed also be numbered.
The Promise here is that the land is going to go to Abraham, and to his seed, "for ever." At this point, there are two meanings the term could have:
- A reference to the singular son, Isaac.
- A reference to the collective of descendents, those who Abraham begets.
That the collective is in view is evident from v.16; but of course, the collective will itself manifest by further generation from the original singular, Isaac (Gen 21:12; Rom 9:7; et. al.). Thus, in Gen 35:12, God can say to Jacob a similar promise, even referencing that Isaac is part of that promise, "And the land which I gave Abraham and Isaac, to thee I will give it, and to thy seed after thee will I give the land." The promise to the seed stays constrained, being given to a specific lineage. But it is also constrained in a different way by Gen 21:12, "for in Isaac shall thy seed be called."
So we see that the #2 meaning above is a special type of begetting from Abraham. Physical offspring are not "called" out (they are generated through sexual reproduction), and so the promised seed that God was referring to with Abraham was to be generated by calling. In truth this is the type of offspring that God had made the promise to, and that He was engendering through Abraham all along (Rom 4:13), to any who would believe (Rom 4:16) the word of God (1 Pet 1:23). Which word started the promise of the seed back in Gen 3:15, and is just being constrained to Abraham's physical and spiritual line in this promise.
It is this call to believe in God's promise that Esau obviously despised and Jacob sought diligently (Gen 25:31-32; Heb 12:16; of course Jacob was trusting in his own devices to attain to the promise, rather than trusting God to make it come to pass without Jacob's help). Thus faith in the promise (specifically, promised ONE) is what differentiates even the physical seed of Abraham (ethnic Israel) from the promised seed within the physical (believing Israel; Rom 9:6-7). It is the remnant of faithful within Israel that obtains the promise (2 Kg 19:30-31).
The physical lineage having the specific promise terminates in Jesus Christ, the seed of David (Rom 1:3), the seed of Abraham (Heb 2:16), who we learn was the terminal point of the promise from Gal 3:16. Christ has no physical offspring, but He does still engender spiritual offspring through the word. And it is this collective seed, those who heed the calling of the word to believe the promise of God in Christ, as Mike quoted in his answer, Gal 3:29...
And if ye be Christ’s, then are ye Abraham’s seed, and heirs according
to the promise.
The spiritual seed is the collective of the faithful (whether Jew or Gentile; Eph 2:14-16), generated by the call to believe the promise of the single physical seed that terminated the physical line of where the promise focused. These are those that will be heirs (again, Gal 3:29; Rom 8:17; et. al.).