Meaning of 'whether it was NOT true'

[Tamara]

Hi

'I demanded of her immediately whether it was not true that… <some fact that was supposed to have taken place (?? or which verb form should be used here??) 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 never (thank you, Amy) used not in 'whether-' clause.)
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It’s impossible to learn swimming without entering the water…

[Alan]

Hi Tamara,

I would rewrite:

[Tamara]

Alan, that's A.Christie...

OK. I understand the point.
Thank you.
_________________
It’s impossible to learn swimming without entering the water…

[Yankee]

Hi Tamara

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:

1. Direct speech: "Is it not true that it happened?"
2. 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.

Amy

[Tamara]

Yes, Amy, that’s the point. Thank you.

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.)

Right?
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It’s impossible to learn swimming without entering the water…

[Yankee]

.
Perfect! In the first case the speaker is likely asking for verification of what he/she already believes to be true.

Go to the head of the class, Tamara.
.