The second one because it's a THIRD CONDITIONAL : IF + PAST PERFECT, WOULD + PAST INFINITIVE ( have + past participal). You use the third conditional when you express an impossible situation

The first sentence would be: If I hadn't seen it with my own eyes, I would never have believed it. The second one is correct.

They mean exactly the same. two slightly different ways of saying the same thing.

All are correct. Just different means.