1. There are 17 legendaries available in Black2/White 2, but you can't have all of them at once if you don't trade/use Memory Link.
2. Cobalion is in Route 13, Virizion is in Route 11, Terrakion is in Route 22, Reshiram/Zekrom are in Dragonspiral Tower (but only if you defeat N in his castle and receive the corresponding stone from him), Kyurem is in Giant Chasm, [email protected]
3. Google it yourself.
4. That's Pokémon Mystery Dungeon to you. From what I can tell you, there are a lot; too many for me to count. Yes, they're worth buying.
5. Getting Black/White would be good, since they let you see what happened before B2/W2.
6. Seaside Cave, no. Reversal Mountain, yes.