Because i have read that some users imported compressed private keys to electrum and the pub adress was a completely different one.
Compresses and uncompressed formats of your private key would generate different addresses.
You don't need to derive the uncompressed format of your private key. The address you are looking for should be derived from the compresses private key. It's not common to use uncompressed private keys.
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?
If you import your private key into electrum, you will see the derived address and you can check if that's correct.
If the address is correct, it means that you have the correct private key.