will the bitcoin core be able to create a wallet.dat file offline
Sure, any software wallet should be able to generate a new wallet without the Internet connection. In fact, some storage setups are meant to be offline all the time.
the bitcoin core will detect the old wallet.dat file which is in the usb flash, the 1 btc old wallet.dat. will it be able to detect the wallet with the balance on it even if it is offline, the bitcoin core here is fully synced to take notice
Wallet.dat contains information about transactions so your balance will be visible while being offline. As for importing your backup, if there are any confirmed transactions associated with that wallet after you made that backup, the balance won't be up-to-date if your online node has already downloaded the blocks in which they were included. You will have to run Bitcoin Core with -rescan parameter. If you copied you wallet file after all transactions have been confirmed then simply importing it should be enough.