Sam,

"So then, these hearers are those to whom Jesus has been preached and
who have heard from the apostles and prophets the actual preaching of
Jesus, and who have responded to that message with faith."

I think that’s likely. The question is whether what Paul writes in his letter applies only to those who have so responded.

Paul says in Ephesians 2 that Christ broke down the dividing wall of the Law that brought enmity between Jews and Gentiles. This was achieved by "the blood of Christ," "in His flesh," through the cross" — that is, through the physical death of Christ. Paul says nothing about Christ’s death being made efficacious only by virtue of his readers’ response of repentance or faith or love. Interestingly also, Paul doesn’t say that Christ’s resurrection unites the Jew and the Gentile into the one new man. Death in the flesh is the great leveler of fleshly distinctions like Jew and Greek. We are all one man in death.

But that’s not the end of the story; oneness in death is only the starting point. There’s a change in tone beginning in v. 17:

And He came and preached peace to you who were far away, and peace to those who were near; for through Him we both have our access in one Spirit to the Father. (v. 17f)

I think it’s here that Paul shifts from death to resurrection, from flesh to spirit. The Gentiles were far away, the Jews were near, but in the crucifixion these distances were erased. Now, in the resurrection, Christ preaches peace to everyone, grants spiritual access to everyone. Again, Paul says nothing about his readers’ response: it is the efficacy of the resurrection that’s the source of this new spiritual life. The dividing wall is leveled in Christ’s death, making everyone equal in the flesh. In His resurrection everyone is "being built together into a dwelling of God in the Spirit."

It’s later, especially beginning in chapter 4, that Paul begins to emphasize his readers’ active participation in Christ’s work. He entreats them to walk in a manner worthy of their calling, with patience and love, in the unity of the Spirit and the bond of peace. This is the new resurrection life that Paul is talking about here.

There is one body and one Spirit, just as also you were called in one hope of your calling. (Eph. 4:4)

The one body comes together in Christ’s death; the one Spirit arises from His resurrection: this is the twofold offer from God. Paul then outlines what will happen as his readers continue to walk in a manner worthy of this offer: grace, gifts, service, maturity. This leads to 4:17, the beginning of the next passage talking about the "new man," and the subject of a separate post. In light of this discussion we probably ought to elaborate a bit more there about the reader’s response.

"Is there only a tiny core of this ‘new humanity’? And does this then exclude the others?"

The thrust of Ephesians is generally this: Christ reconciled everyone in the body of His death; Christ offered new life in the Spirit through His resurrection; Paul encourages his readers to respond appropriately to what Christ has already accomplished. Does Paul propose that a new distinction be made between Christian and non-Christian? Is he collapsing the old distinctions between Jew and Gentile, near and far, microcosm and macrocosm, only to open a pathway to a new microcosm along a different axis, leaving a newly-configured macrocosm separated from God outside a newly-erected dividing wall?

In Chapter 5 Paul does invoke certain threats about the loss of inheritance in the kingdom and about the wrath of God. And in 4:17 he encourages his Gentile readers not to live the way they once did, in a life excluded from God. But the thrust remains consistent: Christ did these things for you, therefore you should respond appropriately to what Christ has already done. If you don’t, then you’re living the life of the "old man," where fleshly distinctions like Jew versus Gentile still hold sway. It would appear that even believers can slip back into the fleshly old man, separated from the spiritual life of God in the resurrected Christ. Does this mean that individual believers can go back and forth between saved and lost, between being Christian and being non-Christian? I don’t think that’s what Paul has in mind here.