As I said you are trying to make an artificial disagreement here. Elsewhere I noted Mounce, who states nearly the same wooden word for word translation (
https://www.billmounce.com/monday-with-mounce/jesus-god-rom-9-5):
This is the same understanding of Mounce, Metzger, Harris, Beza, etc.--that ὁ ὢν is the functional equivalent of ὅς ἐστι. It's not just with this participle, but
the attributive participle itself would most normally recognized as the functional equivalent of a relative together with a finite verb.
Both are simply saying the same things your grammars will tell you. The translations they provide are
not agreeable to the idiom of the English language, thus Gryllus calls it "rather awkward English" and says "we have to do something with it
to capture the sense of the Greek in good English." So as I have
repeatedly stated the attributive participle and the relative clause in Greek are agnates (have equivalent function), but since it has no direct English equivalent the next best option is the relative clause. A relative in this case is to be preferred over an anaphoric usage of the English article accompanied by a relative. Otherwise, it is redundant and you've entirely missed the point of the adjectival construction. You could be taught this, if your cup wasn't already full.
The attributive participle here
is in the second attributive construction. But you and TRJM have a very narrow concept of how the basic constructions work in more complex sentences. That's been shown from many grammars and numerous examples, and both of you are just playing rule makers. Even if we did take it as "an appositive," you are splitting hairs because the only difference is that ὁ Χριστὸς and ὁ ὢν ἐπὶ πάντων θεὸς would have equal semantic connection to the rest of the sentence, and the translation in English would be unaffected.