The first two cases you cite - ἐξ ὧν, ὧν οἱ - are instances of different declinations of the pronoun ὅς. In the third case - ὁ ὢν - ὢν is the participial form of the verb "to be" (εἰμί)
Chrysostom was not oblivious to this: he simply did not touch on it in his homily, since it was something that was already well understood (see below). In his 16th homily, he comments on verses 4-5 together and chooses to dwell on verse 4, rather than verse 5:
To whom pertains the adoption, and the glory, and the covenants, and the giving of the Law, and the service of God,and the promises; whose are the father's, and of whom as concerning the flesh Christ came, Who is over all, God blessed forever. Amen
And what is this? One asks. For if with a view to the belief of others he was willing to become accursed, he ought to have also wished for this in the Gentiles' behalf. But if he wishes it in the Jews' behalf only, it is a proof that he did not wish it for Christ's sake, but for his own relationship to them. But in fact if he had prayed for the Gentiles only, this would not have been equally clear. But since it is for the Jews only, it is a clear proof that it is only for Christ's glory that he is thus earnest. And I am aware that what I am saying will seem a paradox to you. Still if you do not make a disturbance, I will presently endeavor to make it clear. For what he has said he has not said nakedly; but since all were talking and accusing God, that after being counted worthy of the name of sons, and receiving the Law, and knowing Him beyond all men, and enjoying such great glory, and serving him beyond the whole world, and receiving the promises, and being from fathers who were His friends, and what was the greatest thing of all, having been forefathers of Christ Himself (for this is the meaning of the words, of whom, as concerning the flesh, Christ came), they are now cast out and disgraced; and in their place are introduced men who had never known Him, of the Gentiles. Now since they said all this, and blasphemed God, Paul hearing it, and being cut to the heart, and vexed for God's glory's sake, wished that he were accursed, had it been possible, so that they might be saved, and this blasphemy be put a stop to, and God might not seem to have deceived the offspring of those to whom He promised the gifts. And that you may see that it was in sorrow for this, that the promise of God might not seem to fall to the ground, which said to Abraham, I will give this land to you and to your seed, that he uttered this wish, he proceeds,
Not as though the word of God had taken none effect.
John Chrysostom, who lived between 349 and 407, somewhat post-dates Arianism, which was condemned at the 1st Ecumenical council at Nicea in 325. The principal champion against Arianism was Athanasius, who lived from 296-373. Arius himself died in 336. Athanasius did, in fact, refer to the verse in question as you suggest as a defense against Arius, in Chapter 3 of his First Discourse Against the Arians.
A catalog of other patristic references on the interpretation of this verse has been given by the late Eastern Orthodox Archbishop Dmitri Royster (of Dallas) in his 300-pg commentary, St. Paul's Epistle to the Romans:
"Christ, who is over all, God blessed for ever" should be understood as "being God over all," based on the use of [ἐξ ὧν], "from whom [came] the Christ according to the flesh," and [ὁ ὢν], "the one Being" or "the one Who is" (hōn here is the participle of eimi, "to be"); thus: "being God over all, blessed for ever [unto the ages]." Ho hōn is written on the icon of Christ; it is related to the Old Testament name of God, "I am"." (Note that theos, God, in the predicate position following the verb "to be" is used with out the article, as in John 1:1; see St. John Chrysostom, On the Gospel of St. John, Homily IV, no. 3 for an explanation of this usage.) In general, the holy Fathers understood this last clause to mean that Christ is God over all (see St. Athanasius *Discourse I, Against the Arians, chap. III, no. 10; chap. IV, no. 11; St. Ambrose, On the Holy Spirit, Book I, chap. iii, no. 46; St. Hippolytus, Against the Heresy of One Noetus, no. 2; St. Irenaeus, Against Heresies, Book III, chap. xvi, no 3, and others).
* pp. 233-34