I agree. The force of attraction between ὁ and θεὸς (and everything in between) is overwhelming, at least by the judgement of the grammarians of old. So ὁ ὢν could only refer to Christ if θεὸς wasn't present.The clause is translated as “God, the one who is above all, be blessed forever, Amen.”
In effect the Trinitarian translation renders θεὸς an adjective to break the force of attraction between ὁ and θεὸς. In any case ὁ ὢν ἐπὶ πάντων would still not necessarily refer to Christ, even if θεὸς wasn't present. It would only necessarily refer to Christ if, as I suggested, ὢν were deleted to read ὁ ἐπὶ πάντων.
So it is the presence of θεὸς, that Trinitarians seek to capitalize on, which renders the sense unambiguously non-trinitarian.
Also ἐπὶ πάντων is only ever used by Paul of the Father, albeit Christ uses ἐπάνω πάντων (twice) of himself in Jhn 3:31 although obviously not so as to exclude God his Father.
So there isn't any theological objection to Christ being ἐπὶ πάντων, or to the Father being ἐπὶ πάντων.
Context is what determines it. In the Pauline epistles, Paul was speaking to Gentiles who weren't familiar with Jewish monotheism, and needed to be taught God's supremacy from the ground up, which is why it is the Father aloneis ἐπὶ πάντων, and not Christ. The Jews would have understood that Christ wasn't referring to himself being above him Father when he used ἐπάνω πάντων.