Even if the Ethiopians attack earlier they're going to face long odds of pulling off a meaningful victory given their plethora of military difficulties.
But let's say they run the table and inflict a decisive defeat on the Italian forces. Badoglio is forced to retreat and the Emperor has scored a major victory. Both sides are bloodied and Badoglio is forced to heavily rely on the Italian airforce to use poison gas on the Ethiopian troops.
Meanwhile Selassie has bought himself some time but nothing more. Despite the poor weather, his armies cannot hope to match the material superiority of the Italian forces. The somewhat Pyhrric victory at Maychew convinces Selassie that his armies cannot defeat the Italians in the field without air superiority so he decides to pursue the course of guerrilla warfare hoping to drag the conflict out long enough for Italy to give up, or accept a negotiated peace.
Best case scenario for Selassie: Ethiopian guerrillas and horrendous weather stall the Italians for the remainder of the rainy season. Selassie uses this and concrete evidence of Italian gas attacks to gain international support for a resistance movement. At the end of the rainy season, the Italian army gets moving again and conquers the Addis Ababa forcing Selassie into exile. However they face a much stronger resistance that continues to be a bigger thorn in the side of Fascist Italy than OTL.
Also the lack of a lightning victory in Ethiopia might increase Italian jingoism as the need to validate the strength of the Italian military will certainly be stronger than OTL.
A longer war also might see increased action or perhaps even strategic material sanctions from the League of Nations.