I doubt the Derg could survive the fall of the USSR, considering that it required extensive military support to keep itself afloat and containing the multiple nationalist insurgencies that were becoming even more powerful with the weakening of the Derg. If the Derg turns towards establishing a mixed economy earlier and embracing reforms, it could survive but there's the issue of the insurgencies - the ELF, EPLF, TPLF, OLF, ONLF, etc. and then the ultimate coalition of the Ethiopian People's Revolutionary Democratic Front. The Ethiopian Civil War took a lot out of the Derg so if its more successful against the various rebel groups, it could focus on the aforementioned economic reforms and acquiring support from other countries to lessen reliance on the Soviets. I would recommend having Tafari Benti come to power, considering he was less authoritarian and favored the establishment of a Popular Front amongst parties like MEISON and the EPRP as well among the civilian Left.
Regarding the Horn of Africa, I'm not sure.