@Removed I don't think you fully understand the popular hatred that got built up among Oromos against Habashis as the rulers of Ethiopia. The Oromo leaders come and go. Ordinary people on the other hand carry the resentment through and over multiple generations.
On your first point, geography was never a limiting factor for self-rule movements around the world. The disputes Oromos have with the neighboring states do not constitute a unified obstacle against them. To the contrary, the relationships among those states are volatile and it won't be that hard to employ their weaknesses for solidify the Oromo plans. Habashi Amhara are not one thing with Habashi Tigray. Somalia is not in alliance with Kenya. Djibouti is not a concubine of Eritrea. Somaliland would rather prefer to save the drowning Yemenis than help Somalis not be eaten by sharks.
On your second point, Habashis ruled Ethiopia. See how far they got in reigning in Oromos. What makes you think that Oromos would be any more successful in yoking Habashis in one Ethiopia now?
Independence is a far better option for Oromos in the long term. Once they get strong as an sovereign country, Oromos can start a push at annexing adjacent territories on their own terms and not on the federalist/centralist utopian under-the-gun state.
My question to you is this: Who is actually in charge in Ethiopia right now? What group?