If Allende had ran again in 1976 and won, the military would have kicked him out of office.
Not necessarily, for that to happen you need the Commander in Chief of the Army to be willing to do so. Schneider might still be around, and he was not going to allow that. His likely successor, General Prats, was also unwilling to oust Allende. And remember, Augusto Pinochet got named Commander in Chief since he knew Allende well and the President considered him as friend, or at least as a loyal, constitutional General (in that regard he was like Franco in 1936, Pinochet could have ended up loyal to Allende with the right set of circumstances). Allende was quite popular up to the beginning of 1972, when the economy collapsed and he alienated the DC. With the economic crisis of 1973 right in the middle of Alessandri's term, Allende becomes much stronger, and he can certainly win a three way race again by a greater margin.