if i import the private key on an offline device with electrum installed, it will instantly confirm if the private key is valid to the belonging pub adress?
On offline Electum, it will not synchronized with the blockchain and you will not see your coins. You know that you have coins there, you can see it through the watch-only wallet which is online and which you can use to create an unsigned transaction, send it to the offline Electum through QR code or USB stick. The offline Electum will only be used for signing the unsigned transaction and you will send the signed transaction back to the watch-only wallet to broadcast it.