You can send your BTC away after 0 confirmations if you export the private key and use any other wallet. This is really easy so everyone should be able to do it.
Yes but then he compromizes the security of the wallet.
Suppost 1 of them goes scammer , the other wallet is still holding, if you expose the private key to both services then you double your risk of theft.
It's not a good idea to mix services just as its not a good idea to use the same password in all websites, its the same logic.
What are you talking about??? Sending a transaction doesn't reveal your private key.