OK first, the numenorians were descended from the edain. Not all, or even most, were half-elves. Just the royal line (and by intermarriage the noble houses). But ALL numenorians got long life.
Luthien's gift was granted by Eru, after Manwe and Mandos consulted with him. Later, he also issued a ruling about whether half-elves counted as elves or humans for the purposes of death and immortality-- that they could choose which heritage they belonged to. Luthien was half-Maia in any case, not a pure Elf. Tuor got to become an elf, but again Eru granted it.
It's "fate" not "faith". Singlular: "Vala", plural: "Valar".
Elves are immortal WITHIN THE LIFE OF THE WORLD. That means, they're alive as long as the universe exists but what happens after the universe ends is a mystery. Humans live only briefly in the world, but have immortal souls that leave the circles of the world and will exist forever. They're more like Maiar. It's known that humans will take part in Eru Illuvatar's creation of a second universe, whereas the fate of elves after the End is unknown.
A side effect of the human gift of true immortality is that our bodies and souls aren't tied to this world so strongly and so die quickly and easily, thence to pass into the world Beyond.
Sauron wasn't challenging Eru or the fate of Men. He was fighting to be a temporal king of Middle Earth. His master, Morgoth, was trying to take over the universe, even the laws of science.
Elves turned evil all the time. The Noldor, lead by Feanor, did many evil things. None joined with Morgoth, but their actions accomplished his will. Morgoth often allowed elves to escape still under his control-- IE on his side. Orcs, obviously, are corrupted elves also. Eol was evil, and his son Maeglin sided with Morgoth and assisted in the sack of Gondolin. And, no, not every sin was forgiven. The sons of Feanor certainly weren't, nor Maeglin... etc.
So, anon, in sum you are an idiot.