Firstly, I’ll check the balance with the uncompressed master public key and the compressed master public key using 2 watch-only wallets. If the balance is different, I’d need to import both WIF private keys. If the balance is the same, my idea is to import the uncompressed WIF.

It is possible your old Bitcoins were sent to pubkey instead of the Bitcoin address. If that's the case, Electrum can't handle "send to pubkey" inputs so you can't use it to sign offline.