'I demanded of her immediately whether it was not true that… <some fact that was supposed to have taken place [size=92](?? or which verb form should be used here??)[/size] some time ago>’
Could you say how the meaning of the question would change if we (I :)) removed not? (…whether it was true that…)
(In my view, the general meaning would be practically the same (because of ‘whether’) and only the inversion in “her” potential answer (‘Yes/No’ vs ‘No/Yes’) could be caused by that.
But the emphasis made (in the question itself) seems to be quite different –
help me, please, to understand it better.
(I have [color=blue]never (thank you, Amy) used not in ‘whether-’ clause.)
Before I give you my opinion about a possible difference in meaning, let me ask you what difference you see (feel) between the two “direct speech” examples I’ve written:
Direct speech: “Is it not true that it happened?”
Indirect speech: I asked her whether it was not true that it had happened.
1a. Direct speech: “Did it happen?”
2a. Indirect speech: I asked her whether it had happened.
In your second case (and Alan’s one) I am/was just interested to know (from a witness) whether the fact itself has/had taken place. Or not
In the first your case I have/had a definite own version (suspicion, evidence, opinion, etc.) and is/was just seeking for it’s (complete) validation. (But am quite sure in it.)