If Alessandri had won the Chilean presidency in 1970

[TX Conservative Dem]

Let's say Jorge Alessandri wins the Chilean presidency in 1970 (with the tactic backing of Nixon and CIA) defeating Salvador Allende. The 1973 coup never would have happened: Pinochet never would have came into prominence 

[Lumine]

If Alessandri had won you might have not seen a happy ending anyway. I personally admire him, but at that point he was tired and was forced to accept being a candidate, and he lacked support in Congress (the National Party distrusted him and there were no independents), so Alessandri is going to find massive gridlock, even worse than his first term. There are some solutions, though:

A pact with the Democratic Christians: Hard to achieve given the prominence of the left wing, but certainly possible if Alessandri gives them a lot of concessions and the left seems to be going radical.

Calling for a new election: Alessandri was also willing to do this, knowing that there were no other options in the right and that the Democratic Christian would have won. Of course, the DC candidate has to be changed then, since Radomiro Tomic (OTL candidate) was from the left wing. Edmundo Perez Zujovic would be an excellent choice.

Trying to survive the term: Could work, but it's not going to end well... The Chilean right was in the middle of a realignment process, and they only reason they never destroyed themselves was because they had Allende as an enemy. Without the DC and with the right in anarchy, Alessandri is bound to rage quit at some point.

Now, yes, Pinochet is never going into prominence here and the army commander is going to be the democratic Schneider (Alessandri victory butterflies away his assassination). The Unidad Popular is a wild card, since after losing three elections in a row there was talk in the Socialist Party of revolution. If Alessandri's term is a failure, they might attempt something (just might), they could contest the next election with a different candidate, or they might try with Allende one last time.

As with any left or right-wing government in 1960's and 1970's Chile, it all depends of the Democratic Christians...

[TX Conservative Dem]

If Allende had ran again in 1976 and won, the military would have kicked him out of office.

[Lumine]

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.