With due respect, Gary, you might have less questions if you pick one thing at a time to ask about, rather than going in many different directions (talking about differences in Christology, then differences in the number of councils accepted, now apparent similarities in liturgical form, etc). You’re kinda all over the place, my friend. It makes it difficult to respond to or make sense of your posts.
I can’t speak for the other OO churches, but it is actually not traditional among the Copts to refer to their liturgies as “Divine Liturgy”. This is something I believe we have borrowed from the Byzantines, and I only ever see it in English anyway. In Coptic, it is just “Ti-Anaphora ente pi-Agios… (Basilios, Kyrillos, Gregorios)”, which literally means “The Anaphora of Saint _____” (Basil, Cyril, Gregory, depending on which is being celebrated). In Arabic, it is usually just “al-Quddas” (liturgy), again followed by the saint’s name (usually Basil).
Also, as you can tell from the above explanation, the liturgy (no matter what we call it) is not just one thing. Both the Chalcedonian and non-Chalcedonian Orthodox do not limit themselves to one form of liturgy, although they both have one form that is used more often than the others during the liturgical year (for the EO, it’s the Liturgy of St. John Chrysostom; for the Copts, it is the Liturgy of St. Basil; for the Syrians, it is the Liturgy of St. James, etc). So there is no single DL. Divine Liturgy is just the Eastern term for what those of the Latin tradition call “Mass”, and just like you have many different kinds of Mass (Latin, Byzantine, Chaldean, etc.), there are many different liturgies used in the East, irrespective of whether we’re looking at Chalcedonian or non-Chalcedonian churches.
Exactly as I have just described it: The churches use the liturgies that are traditional to them. This is no different than in the Roman communion, wherein a Latin Catholic in Italy may use the Ambrosian rite, while a Latin Catholic in Spain may use the Mozarabic. We don’t ask how this “works”, or pretend that this is some kind of problem for the unity of the Roman communion, because it is not. Please extend the same courtesy to the East and the Orient. We are divided over questions of Christology, not liturgical use (after all, the EO do also occasionally use the Liturgy of St. Basil, if I’m not mistaken). This is a non-issue for the people who are actually Orthodox.
This has nothing to do with anything that you wrote before it, though. Even if, hypothetically, communion were restored between the non-Chalcedonian and the Chalcedonian Orthodox, it would mean nothing for the prospects of union between Orthodoxy and Rome, as the issues that separate Rome from both the OO and the EO are not at all the analogous or comparable to those that separate the OO and the EO from one another (as Cavaradossi has already explained).