U have posed two important questions. First, as u said, these opposition groups are mainly ethnic based. But u have to put that to the context of Ethiopian political history. Ethiopia knew for a very long time class society with extreme feudalistic approach. In the fourteenth century, Emperor Amda Siyon made a serious effort at expansion, annexing territories and consolidating these into an Ethiopia which more or less conformed to the boundaries of today. To do so, he suppressed ethnic movements and firmly established Ethiopia as an Amharic and Christian nation. He granted a form of autonomy to regional rulers as his feudal vassals with authority over gults, or fiefs. These were mostly puppets who never bothered to serve their consistencies. It is only during the Derg;s revolution, things have changed, at least on paper. This long history of ethnic oppression and marginalization has left a scar on the Ethiopian society. Today, the degree of trust among various ethnic groups is extremely low. No leader has so far succeeded to rule Ethiopia without using a gun barrel. The fact that all opposition groups now are ethnic based reflects this unfortunate historical reality. Nonetheless, once all organization grew up in their separate ways, it is easier for them to merge. At the end of the day, they all want to see a nation where equality of all is genuinely respected.
As to ur second question on article 39, the answer is easy. TPLF had always the dream of creating Abay Tigray, by chopping off territories from here and there. In TPLF crab mentality, Tigray will never sustain as a ruler of the rest of Ethiopia. They think they know deep inside they cannot rule Ethiopia for long, being as a minority regime (even if all Tigrayans support TPLF, they all constitute 5% of the population). So they had to come up with alternative plan in case things go ugly. To secede from the rest of Ethiopia legally. Article 39 is included in the constitution only for this purpose. To invoke the right embodied under that article, u need the approval of 3/4th of the parliament. Now, look at the composition of the Ethiopian parliament and tell me if the Afars can in fact successfully invoke Article 39 to secede from Ethiopia?