The relic crown doesn't unlock anyone. Catch all the legendaries, complete white forest, battle subways, collect all 693?. Medal rally. That'll keep you busy for a while
You can get Volcarona by catching it at Relic Castle or making your Larvesta (if you have one) evolve into Volcarona starting from level 59.
Relic castle
The Relic Statues in both Pokemon Black and White are acually Darmanitans. But these Darmanitans have the secret ability Zen Mode which will change it's form and stats when its HP is low. You can wake them up with the RareCandyBar that Prof. Juniper will give you when you enter.
Petilil evolves with a Sun stone, not by level. You can get one in Nimbasa City, as well as in the Relic Castle. Hope it helps
On Pokemon black, both fossils are in the relic castle on the entrance floor, but you can only pick one fossil the cover fossil which is the turtle Pokemon and the plume fossil is the bird Pokemon.
From a backpacker in the relic castle (Only 1 of 2: the Plume fossil for Archen or the cover fossil for tirtouga) and a miner in Twist Mountain (1 per day)
4 because the sell pokemon white and pokemon black and know they sell pokemon white 2 and pokemon black 2
Yes
There are 300 Pokemon in the Pokemon Black 2 and White 2 Unova Pokedex.
Route 20 does not exist in Pokemon White, but does in Pokemon White 2.
Reshiram is not in Pokemon White you need to trade either with Pokemon Black or Pokemon White 2.
White Kyurem