I'll repost my anwser from a different
thread.
For Czechoslovakia to keep 1918 borders would require butterflying away not only Munich, but WW2 as well, so that it can keep Transcarpathia. Federalizing Czechoslovakia on the begining of its existence would be a good start, so to satisfy the Slovaks demand for self rule, as well as diminish the pressure from the Germans, Poles and Hungarians. So lets say that the first constitution from 1920 sticks to the early declarations of the Czechoslovak Committee in Paris, and the country forms as a federation of Bohemia, Moravia, Sudetenland, Tsesin, Slovakia, Transcarpathia - thus the Czechs and Slovaks are still the most important nations of the state, while the others have more self-rule. This might be achieved by having Stefanik survive and rallying the Slovaks during the period of the country's formation, demanding autonomy for Slovakia, leading to Prague folding to his demands, as well as the demands of the Germans and Poles.
Without the Tsesin issue, and probably without Benes in charge, Czechoslovakia stands a better chance at forming an alliance with the other countries of central Europe - for example, allowing for the unification of the Little Entente and the Warsaw Accord. If ITTL Hitler still comes to power, Munich is different as the Sudeten Germans movement is weaker, their demands for autonomy met (and many possibly prefering a democratic Czechoslovakia to fascist Germany). Czechoslovakia also has much firmer guarantees. So the leadership refuses German claims, Hitler starts the war and loses Fall Grun. Czechoslovakia survives, the victory over the Germans allowing it to maintain unity, national ethos being something like "together we can defeat anyone", "brotherhood and unnty", etc.
