Yes.
You have to go to the top of mt conoret, where you'll have a battle with your rival against commander Mars and Jupiter. After that you'll go to the distortion world where you have to beat Cyrus and fight Gratina. Once all that is completed, team Galactic will be gone from mt Coronet. To find Cyrus and the top of mt Coronet: http://www.mahalo.com/Pokemon-platinum-walkthrough-part-6/
There is a map of Mt. Coronet in the Sinnoh region.
yes your rival will heal your Pokemon before you fight Cyrus.
There is no Mt. Coronet in HeartGold as it does not take place in the Sinnoh region
at the top of mt coronet(arch=spear pillar)
Top of Mt Coronet? It's all in what he's said.
He's in the Galactic Veilstone Building, and on top of Mt. Coronet. But not after he leaves Mt. Coronet.
You can get Absol at the top of Mt. Coronet.
follow the path
Yes.
If you're playing Diamond, the Pokemon in Mt. Coronet should be Dialga. You have to get the 7th badge before you are able to get to the top of Mt. Coronet where Dialga is though.
you don't you have to go around it
On top of Mt. Coronet.
no idea
there is only one mt coronet but there are four entrances. you can get to the top of mount coronet through the oreburgh entrance
You have to beat the seventh gym and get up their.