you need to time travel with the statue where you first met celibi, and you can do ranger missions in the past. after a few missions you break the tangroth slate, and if you get to the place it actually is, it breaks and you can go in there.
good luck :)
It is in the Sky Fortress.
he is in the sky
rasp cavern
no regigigas does not
pokemon originaly came from there mother
Its a wobbufet which you get to capture in the game itself without opening the door.
yes
in the cave of icicles
you never use it
it is in daybreakruins i think
you go there at the start of the game
never after shaymin its giratina