@Philip No, that is not my opinion. My view is that, Nestorians would say that their position is the true and orthodox position. If you call one group the only oriental and orthodox group, then would you not exclude Nestorians and thus marginalize them? Nestorians for instance would say, that rejecting Ephesus is the orthodox position...
I think you are making a mistake in assume there is a value or theological judgement embedded in his word choice; perceiving the word "only" where it wasn't intended. Referring to one group as the "[X] Orthodox Church" does not prejudice the orthodoxy or un-orthodoxy of any particular position.
The big POD here would be if they are able to proselytize to the Horn of Africa. If so, then the entire Indian Ocean trade network becomes available for Miaphysite Christianity to spread. Islam was extremely successful at using this network in OTL.
There are factors that go against this, though. Ethiopia is mountainous and politically, was not unified really for a long time. It also had pockets of Jewish practitioners that remained remarkably resistant to conversion. Ethiopia was never particularly rich, either, and that prevented it from using merchants to spread the faith.
As for the main question, I think this is a good stepping-off point for discussion. Ethiopia's geography has at once been a curse and a blessing: it has inhibited inter-regional trade and the construction of road networks, limited the amount of arable land, precluded the availability of navigable rivers, and so forth, but it's also what has made the country so resistant to conquest (cf. Afghanistan). If you want a merchant-based proselytization, you need to restore the country to an Aksumite-era level of capacity, when those trade networks did exist. This might be easier than you might think: the state was fairly centralized for long stretches from the 12th to 16th centuries and then again during the Gondarine period in the 17th and 18th centuries. If a stronger Solomonic empire is able to definitively subjugate Ifat by the 14th century and secure a reliable coast-line, possibly from Massawa to Berbera, it might have a
shot at becoming a player in the Indian Ocean trade again. It might then hop south and seize control of the proto-Swahili coast and spread the religion inland from there. (I've thought of a timeline based more or less on this premise, but I doubt it'll ever be written.)
There is an alternative: if Ethiopia is able to spread its influence into the declining Alodia and Makuria to the West, establishing "protectorates" over them, it could be viable to spread Orthodoxy west along the Sahelian trade routes, although it would pretty quickly run into resistance from Islam. Still, the Sudanese kingdoms could provide a solution to a long-standing problem, the lack of an episcopal hierarchy due to the Ethiopian Church's immediate subjugation to Alexandria. While there was only one bishop at a time in Ethiopia (who was sent from Egypt), Makuria alone had seven, so it's not unimaginable that an Emperor, having secured suzerainty over Makuria, would order its bishops to consecrate native Ethiopian bishops and effectively establish an autonomous church centuries before OTL.