Any question answered...

Is it possible that there is an undiscovered second earth planet on the earth's orbit, just mirrored 180°, so it's impossible to spot from here - as it would always be hidden by the sun?

Or would Voyager space probes and the likes definitely have discovered it already?

There aren't really any other planets. There's a large dark shell around the Earth, beyond which a large fire burns. What we think are planets are really just holes in this shell through which you can see the fire. The sun is a particularly large one. :)