Thank you for the excellent information! But this is where my own confusion now comes in... I'm assuming these DNA tests are too broad and general; maybe not very accurate (I have thus far taken the Ancestry test, will be doing the 23andMe test next) because for my mother's side it shows that I am 15% Greek/Italian (they consider Greeks and Italians the same thing) 24% "Middle East" which can include Saudi Arabia, Iran, Iraq, UAE, Syria, Jordan, etc. and 6% Cameroon/Congo.
So I was assuming that the Tigrinya people, if you go far enough back, are actually Arab/Congolese mix? Where the Arabs migrated down into Eritrea while the peoples from the Congo were migrating up to Northern Africa? In any case, I will be getting a "second opinion" when I do the other DNA test soon... but I do encourage everyone to do a DNA test, as you just might be surprised at what you find! (We were always told my Father was Irish and Scottish, the DNA test says mostly Irish and Scandinavian instead, which makes sense, since the Scottish were made up of Celtic/Irish and Scandinavian people.)
Forgive me for answering late. I had completely forgotten about this thread.
I am not qualified to answer your questions. However, based on my research of Eritrea's ancient history, I am of the strong opinion that Eritreans look the way they do not from mixing with Middle Easterners but because of their diet (teff) and mountainous topography. I think groups of people naturally begin to develop unique geographical phenotypes when isolated from other groups of people like the Indian sub-continent, East Asians, San People from Southern Africa, Europeans, etc.
Although some mixing always takes place with neighborly people I don't think that was the main factor that makes Eritreans look the way they do because people in the Horn of Africa with almost no contacts with Arabs like the Oromos look similar to the rest of the groups in the Horn of Africa that had some contacts with Arabs.
Mixing is also a two-way street. Horn of Africans are usually reminded they are mixed with Arabs (without evidence I might add), while Arabian Arabs are almost never reminded of being mixed with the Horn of Africa people. I believe this is because the colonial era historians who started this mixing myth couldn't believe the African people in our region could develop such sophisticated civilizations without intermixing with white Arabs. In other words, these are colonial era ideas that were intended to give credence that Africans were 'primitive' people who lacked the capacity to build civilizations without the aid of 'superior' whites.
I think Tigrinya people are African people. They are neither Congolese or Arabs. Their DNA is unique to its region in much of the same way the San People of Southern Africa, who are very light skinned and have unique features, developed in their region from other Africans.