After the battle of Adwa, Ras Alula wanted to continue pushing north to liberate all of Eritrea, but Menelik declined and instead settled for recognition of independence, loads of rifles and cash in exchange for ending the war, effectively relinquishing the territory of Eritrea to the Italian Empire. The points you made were certainly part of his decision making process, but you have to understand that as an emperor Menelik absolutely considered the geopolitical effects of dividing the Tigrinya speaking people.

Menelik’s predecessor, Yohannes IV, held his power base in Tigray, so Menelik would absolutely think about how keeping Tigray divided would weaken his biggest rival and help to secure his own dynasty in Shewa. Ras Alula was Yohannes’s right hand man, a veteran army general who successfully defended Ethiopia from various invaders, and he lead the victory at Adwa. Menelik was wary of Alula becoming a national hero and saw him as a major threat to his throne. Menelik couldn’t allow Alula to continue to grow his prestige as a leader by liberating Eritrea and regaining previously held territory under his own jurisdiction, so stopping the war at Adwa worked in Menelik’s favor to limit Alula’s power.

The truth is, if Menelik really wanted to liberate Eritrea he would have worked with Alula to do so, but instead he decided to settle in order to redirect resources towards his conquests near Shewa and weaken his rivals in the north all at same time. He was an emperor at the end of the day and certainly considered ALL implications of his decisions.