Well, it would make the entire story arc about Hera pointless.
For one thing, since the known cylons initially staged the relationships between Helo and Athena because they were unable to produce children among themselves.
Another reason is that Athena survived disease on the basestar in "A Measure of Salvation" early in the third season because she had conceived a child with a human, which somehow made her immune. Had Helo been a cylon, that could not have happened.
And I should correct myself. I thought the same episode established why Roslin cannot be a cylon, because she met one of the infected cylons and yet did not contract the disease. I was mistaken, though, because I forgot that Baltar injected Hera's blood into Roslin during "Epiphanies" half-way through season 2, so naturally she would be just as immune to the disease as Athena turned out to be.
And if you think it's a plothole that Tigh, Tyrol, Tory and Anders were not affected by that disease, then nope - it's not, because conveniently (and perhaps a little too conveniently), none of them ever met an infected cylon in that episode. The same disease also establishes why Baltar cannot be a cylon, since in the preceding episode, "Torn", the cylons sent him to investigate the disease on an infected basestar precisely because he was immune as a human.
All of this is debunked by the fact that the final five are close enough to human themselves that they can manage to have children with other Cylons (RE: Tigh+Six). This outright refutes your first point. Accepting this, it's easily possible that they also have the same immunity. Because the final five are different.