What's the difference between a half-truth and a half-falsehood?

According to Merriam-Webster, a half-truth is 'a statement that mingles truth and falsehood with deliberate intent to deceive', yes, but a bit of searching shows that on-line dictionaries don't have an entry for half-falsehood, and so, in the lack of this definition, the first thing to come to my mind is that one could define it the same way a half-truth is defined 'a statement that mingles truth and falsehood with deliberate intent to deceive'.

Maybe it is so, however a doubt arose as to whether in a half-falsehood there is a 'deliberate' intent to deceive.

I don't know why, but I'm under the impression that one utters a half-falsehood in order to save themselves rather then to deceive other persons, let alone to deliberately deceive them.

Therefore, what's the difference between a half-truth and a half-falsehood?

One should understand that to call something a falsehood is not the same as to call it false. As Merriam-Webster has it, a falsehood is

1: an untrue statement : lie

2: absence of truth or accuracy

3: the practice of lying : mendacity

That is, two of the three listed meanings explicitly indicate deception. So in theory, both a half-truth and half-falsehood could refer to something said that mixes truth and falsehood with the intent to deceive.

In practice, however, half-truth is a set phrase, and just as English speakers would not use downside-up and outside-in for the conceptually equivalent upside-down or inside-out, half-falsehood has no currency.

Someone who tells a mix of true and false things with no intent to deceive believes they are telling the whole truth. But we might say of the statement:

They were half-right.

None, other than the fact that half falsehood is clumsy and not something you'd ever hear an English speaker say.

It's similar to a glass half full or half empty. It depends one how you look at that kind of situation, optimistically or pessimistically.

With half-truth and half-falsehood, use of one or the other indicates which end the person might have been coming from. In general use, half-lie might be more familiar when coming from the other end of truth.

He tried to convince himself that it wasn't a blatant lie. It was only a half-lie.

She knew it was only a half-truth but she contented herself with the thought that it was true enough.

As others have said, half-falsehood is not a set phrase, so it jumps out. There could be occasions when a writer would want to use it for that very reason, but those occasions would be rare and the context and tone would have to be well-crafted if it is to jump out in a good way.

I think it's evident that you have invented the term, and the concept, of half-falsehood, for some purpose of your own. What purpose that might be, I cannot say. I will say it is the sort of thing one might do in discussion in a philosophy class, more commonly than in a linguistics class, I would think, but I'm not comfortable with the idea of inventing terminology that simply isn't a part of the language and then asking what it means. Why not invent any word you like and ask what it means? Is that a valid way to use this site? I would say not.

How about neologism? That's a valid word. Do you know what it means?