Can you help breakdown 言わん?
She answered as if it was a matter of fact.
Are these incorrect, have different meanings?
Do you know another way of saying "as if to say"?
Edit: In light of Tsuyoshi's comments below, I am changing my etymological explanation.
To answer your first question: 言わんばかり was originally 言わむばかり. This can be broken down into two parts: 言わむ, which is the classical form of 言おう, and ばかり, which here is being used in the sense of something just about to happen. So 言わんばかり might be literally translated as ‘as if [he] wanted to say’.
As for your alternatives:
This is somewhat ungrammatical, but would probably be interpreted as the exact opposite: ‘She answered just saying “of course”.’ If we change it to 「彼女は当たり前と言ったばかりに答えた。」 it would mean, ‘She answered just because it was said to be obvious.’
This would probably be interpreted like the above: ‘She answered just because it was obvious.’
This means the same thing as the original sentence, but is more emphatic. The なんだ can (should?) be omitted.
To further elaborate on Zhen Lin's learned reply, according to 大辞林, "ばかり" derives from the verb "はかる", to measure. Also, considering that one of the meanings attributed to "ばかり" is "approximately", it seems reasonable to say that the listener is imposing a judgment on, or "measuring" the speaker's intended meaning, In this case, it might be more useful to think of "ばかり" as more or less synonymous with "程". In English, it might be something like "She didn't say as much as that it was obvious, but . . . (she might as well have said that it was)".