If they had decided not to use "Miracle on 34th Street" as the title for this film, the best alternative choice would have beenget ready"Sanity Clause"!

The phrase that you're looking for is "working title".
The "original title", in the sense of how it should be listed in movie databases (which is what prompted the original post), is the one under which it was first released.

In the case of
Miracle on 34th Street
, the original title is the one that was on the script as first written. It's not at all unusual for changes to be made to a script, even after shooting has begun. If you've ever seen an actual script, you know that amended pages are printed on colored paper. That's so an actor, director or anyone else who needs a copy of the script doesn't accidentally keep the old pages and throw out the new ones after changes are made.
If you look under "Trivia" right here on imdb, it says that when Maureen O'Haranot yet hired to play Doris Walkerfirst read the script, the title of the movie was
The Big Heart.

The Big Heart was the original title of the movie
No it wasn't.
The "original title" of a movie is the one that it is first released under in its home market, not the one used in some foreign markets.

It appears that the correct title has been restored.
"Everyone is ignorant, only on different subjects". Will Rogers (1879-1935)

The Big Heart was the original title of the movie, and it was released under that name overseas because "34th Street" would mean nothing to non-Americans.