1) i tried importing my main btc address using bitcores command line function, just the adress, not the key.
What command did you use for that?
It used the address as a "label" and associated another address to it... has bitcore now created a different key for that original address i imported...
It sounds like perhaps you used the command:
getnewaddress
If so, then you did not import an address. You asked the wallet to generate a new address of its own with its own private key, and then you created an account associated with that new receiving address. You used your pre-existing address as the name for the account. You could have just as easily named the account "MyNewAccount", but instead you chose to use a bitcoin address that is not associated with the wallet at all as an account name. That is likely to be confusing.
2) i exported some receiving addresses but can't find the keys in wallet.dat or the csv file? Is this correct?
What command did you use to export the addresses?