1. Proof: Can we prove that Native Americans are not descended from Semitic peoples? yes -- all the proof: historical, linguistic, archaeological, anthropological and genetic point to that
in contrast -- can we prove the there was no flood that threatened to wipe out mankind? -- there is no proof for it, but none against. And specifically tot he point that every animal species -- the Bible says in Genesis 6:20 Two of every kind of bird, of every kind of animal and of every kind of creature that moves along the ground will come to you to be kept alive not every animal, so of course not the fish. Nor does it specify that every animal species presently on Earth is descended from these, though it is implied.
2. Relevance to dogma -- Let's take Noah's ark's story here. If is it "shown incorrect" does it in any way change Jewish or Christian dogma? No, not really. Is it central to dogma? No.
in contrast to Mormonism, the idea of Semites in pre-Columbus America is central to dogma as that's the theme of the book of Mormon.
Yes, I agree. But the inerrancy believers on FR do not. They have told me that if even one sentence of Genesis is not literally and exactly true, then Christ is a liar and his sacrifice a sham.
So just as the LDS have had to tap dance their way through archaeology to try and keep the Nephites and Lamanites in the historical records, so have the YEC's had to do much the same thing with geology and astrophysics.