'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.)