@Algr and
@Rive I think there is a fair amount of talking past each other here, but as it reads I agree with the statement that started it, and later logic discussing it via comparison to Newton:
Algr said:
Wanting to disprove Einstein is a noble goal... if you can actually do it.
[separate post]
Ask Newton what .8c + .8c is.
Rive said:
You can't disprove anything what's working. That's just the complete misunderstanding of how science works.
IMO, it's fair to speculate that Newton would answer 1.6C and it is fair to call that "wrong". Broader, it's fair to say that Newton's theory "works" in its domain of applicability and in a handwavey way "works" in the domain in which we use it. But when you do that I think you need to acknowledge that Newton
didn't know what that domain of applicability was and almost certainly didn't have a clue as to just how limited/wrong his theory was. Scientists only realized it later when their measurements got more accurate. Newton probably would have said his theory was "working", because as far as he knew, it was -- but he'd have been wrong. And he probably believe his theory was The Correct One.
I do not think it is fair to call Relativity an "extension" or "redefinement" (refinement?) of Newton's laws; it's a
replacement (for the purpose of theory; practical use is different). The domain of applicability of Newton's laws is very, very small and we mostly use them today in areas known to be
outside their domain of applicability because the math is easier and the error is small enough not to worry about. Their domain are very limited, even single-point special cases at best.
Einstein's theory is much more advanced, with much better experimental data/observation to back it up. But that data isn't infinite in its coverage or precision. There exists the possibility, however remote, that somewhere in those error margins, an experimental result will contradict Relativity. And if that happens, Relativity will be "wrong" and "disproven". Maybe not completely, but within the bounds of the experimental result. In that case, a boundary will need to be drawn or a new theory that matches Relativity's results up to that limit but deviates outside that limit will be needed. Yes, it's extremely unlikely, but if the possibility was absolute zero, then there's be no point in pushing the accuracy of the experimental verification any further. The expectation is that Relativity is 100% accurate in its domain, but it is recognized that it isn't 100% proven, nor can it ever be (though perhaps someday scientists will stop bothering to try to disprove it).
Rive said:
But then you need to prove that within the (proven) old frames your new approach is mathematically equivalent with the old theory.
In my opinion, this is a common statement but an over-statement, and contradicts your previous judgement of the velocity addition example: The velocity addition formulas are not mathematically equivalent. Newton's is wrong, full stop. This is binary. The fact that we use it anyway doesn't mean it isn't wrong.
Extending, the statement then assumes that Relativity is
completely accurate instead of acknowledging that by nature experiments can only provide very good approximate verification. I don't think it's hairsplitting because, again, if Relativity were accepted to be 100% accurate, there'd be no need for further testing and the...non-mainstreamers...would be right to point out that we're considering something 100% accurate without 100% accurate experimental verification. A new theory must agree with the existing body of evidence, not the existing theory (except where the two theories and the evidence overlap).
All that said, it's likely the OP doesn't know the overlap and so doesn't know their theory actually does contradict existing experimental evidence.
Note: I define "domain of applicability" as the domain where a theory is believed to be accurate/correct. It does not include the domain where it is known to be wrong but we just don't care because it's close enough. In that sense, as far as I know,
Newton's laws of motion have no domain of applicability at all. There is no speed where the Galilean velocity addition formula is believed to be accurate. No speed where it and Einstein's formulas give exactly the same result.