Seed and offspring—single or plural?

Galatians 3:16:  Now the promises were made to Abraham and to his offspring. It does not say, "And to offsprings," referring to many, but referring to one, "And to your offspring," who is Christ.

In his letter to the Galatians, Paul is clearly referring to Genesis 22:18:  

  … and in your offspring shall all the nations of the earth be blessed, because you have obeyed my voice.

In this text, Paul rightly takes notice of the fact that Genesis 22:18 refers to Abraham's singular offspring rather than the plural.

However, the Hebrew word zera (seed) that is used In Genesis 22:18 is a collective singular noun. That is to say, even though it is singular, it refers to the entirety of Abraham's offspring  and not a singular person. Therefore, the "seed" (a collective singular noun) refers to the nation as a whole (cf. 1 Pet. 2:9). And this can cause some confusion. How can Paul be correct?  

First, in its original context the "seed" refers to Isaac and not multiple offspring. However, the future tense in Genesis 22:14 ("On the mount of the LORD it shall be provided") establishes that a future fulfillment was in order, which wasn't accomplished by Isaac. Thus Genesis 22 expects a later "seed" fulfillment!

Second, seeing the collective singular as a singular person is the same way others in the Old Testament viewed it. Psalm 72:17 states, "May his name endure forever, his fame continues as long as the sun! May people be blessed in him, all nations call him blessed!" A similar interpretation is observed in Psalm 89, where the psalmist is remarking on 2 Samuel 7:12 and repeatedly uses the singular noun to refer to David's descendants (Psa. 89:4, 29, 36). Therefore, Paul is being consistent with the psalmist in his interpretation.  

So, the apostle Paul is correct in his interpretation that seed = Christ. However, we should note that Jesus has many adopted children – as the number of stars in the sky (cf. Gen. 15:5) and the sand on the seashore (Gen. 22:17). In Galatians 3:29 Paul writes, "And if you are Christ's, then you are Abraham's offspring, heirs according to promise." Christ has one body called the church!  Furthermore, he writes elsewhere, "For the husband is the head of the wife even as Christ is the head of the church [multiple people], his body [singular body], and is himself its Savior" (Eph. 5:23).