I'm not sure about the cultural makeup of modern Eritrea/EU3 Massawa in the EU3 period.
The whole area was (and is) much more complicated culturally than is represented in the game. The trouble is that, for purposes of game mechanics, each culture should have a country tag associated with it (or what's the point?), and while there were many states in this area over the EU3 period, most of them (except Ethiopia and Adal) were small and not very long-lived. In general, it would be realistic to give each Muslim and pagan province in the area its own culture and its own one-province-minor potential rebel country, but do you really want to do that?
Ethiopia should definitely have forts. (Then again, I think that all provinces should have forts, both representing local troops - it's easier to gather armies in your home territory than it is to support an army in foreign territory, so if anything forts should be available at lower tech than land units, because all peoples could and did build forts, and because the AI doesn't understand the implications of not having forts). I'd also make Ethiopia a Feudal Monarchy, and Adal a tribal federation (to reflect the fact that they were a loose and unstable alliance of many different Muslim tribes and city states.) I'd class Tigre province as Orthodox.
If you want to get ambitious, you could add an "Oromo" culture and an Oromo state (as a possible revolter - just copy/paste Zimbabwe and change their culture, religion, capital, government to tribal federation, max decentralized and max land), make all the pagan provinces in the Horn of Africa culturally Oromo, and make Ethiopian culture its own culture group. That would reflect the problems the Ethiopians had maintaining control over their empire. After 1550, the Ethiopians had a lot of trouble with "invading" Oromo, but because the Oromo didn't keep/leave much in the way of written records it's hard to get a sense of what really happened there (where did all these invaders come from?) The way the Ethiopians tell it, it was like Ethiopia was Rome falling to the barbarian hordes.
Finally, you could add decisions for Ethiopia to convert to Catholicism and to convert from Catholicism back to Orthodoxy, as they actually experimented with that. (They had a lot of problems with the experiment, as you might imagine a Catholic government with a mixture of Orthodox, pagan, Muslim, but no Catholic, provinces would.)