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Однажды давным-давно is just like more solemn expression, in the beginning of the fairy-tale for example. It rather can’t be used in the spoken language.

Когда-то – once upon a time (in the past) or ...it will be a long time before… (in the future)

No difference at all. There is even an old (and not funny) joke. Что опять? Не опять, а снова.

You can use она as a substitute for машина.
Это не большая машина.
Она не большая.
Now both...