Dear me, you really think those are interpolations? That is something of a joke in scholarly circles. Especially since it would basically mean that Gregorys whole theology, from the ground up, as unfolded in De anima et resurrectione and De hominis opificio and the Great Oration and the Psalms commentary is an interpolation. Maybe Gregory never really wrote anything (rather like the Oxfordian hyposthesis about Shakespeare). Something similar is true in Isaacs case. And those two are far from being the only patristic universalists; both of the very distinct Alexandrian (including Cappadocian) and Antiochene tradition are full of them, from the days of Pantaenus to the 13th century writings of Solomon of Bostra. Goodness, there are almost overwhelming reasons to believe Gregory Nazianzen, and even Athanasius and Cyril of Alexandria, were so disposed (Gregory unquestionably, really).
And, had our our Lord spoken of everlasting punishment, that would be an interesting argument. But he did not speak English, and in fact did not speak Greek; and the Greek text of Matthew 25:46 (which is the only one you can have in mind) has been read by a great many Greek-speaking and Syriac-speaking fathers, from the earliest days, as saying nothing of the sort.
As it happens, I number among my friends and acquaintances some of the greatest scholars of Orthodox canons and councils and history in the world; and to a man they would assert that the Orthodox Churchno matter what the inclinations of its catechists and prelates may have been down the ageshas never definitively condemned universalism as such, or even addressed it under any synodical or conciliar conditions of special import. It has condemned some teachings that are also, as it happens, universalist. But the sort of universalism found in Gregory and Isaac, which fully acknowledges the reality of judgment and hell, has never even been addressed.
But lets not pursue the issue. Be wrong in good conscience, and by that you shall be saved.