How is "die" used here on it's own?
The same way as "they" in the translation. "die" refers to someone unspecified (plural).
It's not an article but a demonstrative pronoun (see christian's link).
and why is "she" not accepted here? Can't "die" refer to a female?
In singular you'd have to use "Ob die das glauben würde?"
oh, of course!!! Thanks! I overlooked the form of "würden". that was a stupid question of me... :-)