Sandino's escape from the National Guard would surely plunge Nicaragua into civil war IMO. With him acting as a central figure opposition to Somoza, Sandino might be able to turn the majority of the country against Somoza especially after he sets himself up as President.
So say a rough guerilla war goes on from 1934-36/37. Perhaps Sandino makes things interesting and overthows Somoza in the mid-1930's. The US in the midst of the Depression doesn't do anything allowing Sandino to consolidate his rule and enact a number of reforms.