Is it possible a Christian Somalia would allow easier Ethiopian expansion there? An Ethiopian state could stand its ground against Arab raids?
Yes. Probably the best POD for both Somali conversion and Ethiopian expansion is for Ethiopia to consolidate its control over Himyar in the 520s or 530s. Control of both southern Yemen and the horn makes Ethiopia the predominant naval power in the region. My suspicion is that, eventually, Adulis would become the capital of such a realm. Interestingly, this probably pushes most of the more northern Arabs--including the Quraish--firmly into the Persian sphere, since Persia viewed Axum as an ally of the Byzantines.
Now, say there's still a rapid growth of Islam, because the Byzantines and Persians essentially exhaust one another with conflicts. But let's further stipulate that, by this point, the Himyarites are firmly wedded to the Ethiopian crown [this almost certainly requires at least some tolerance for Judaism after the initial conflict between Christian and Jewish Arabs breaks down, and also requires butterflying Abraha's rebellion]. The newly powerful Islamic forces run into difficulties conquering southern Yemen, because the Himyarites are prepared for their style of fighting, they receive reinforcements from Axum, and the still substantial local Jewish population decides the devil they know is better than the devil they don't. However, they do lose territory, and are limited to the coastal cities that can be reinforced easily. None of these butterflies prevent the Muslim forces from rolling through Persia, which would almost certainly still happen. The big question is whether Axum can, or would, help prevent Egypt from falling. To avoid a wank, let's say Egypt still falls, leaving Axum boxed in on 2 sides but still a dominant force in the red sea trade.
Back to the Somalis. Given the perceived need an Adulis-centered Axumite empire would have to defend East Africa against Arab Muslim incursions, they will probably try to expand both trade and their navy. Trade means a possible expansion southward, with coastal Somalis playing a role. So the TTL equivalent of Kiswahili would be a Geez-based creole, not an Arab one.
Beyond that, it's kind of hard to speculate. How long can they survive, and keep Yemen out of Muslim hands? Probably not indefinitely. But can they maintain a navy strong enough to prevent large-scale Muslim invasion across the strait? Again, hard to say.