Any chance you can point us to this address that was funded by millions of BTC? I realise asking you to sign a message is moot if you don't know how to import your private key into a wallet, but pointing out the exact address might be a (small) first step?

This being said, it might be a good idear to create a non-HD wallet using bitcoin core and see if you can import your key then? AFAIK, you can't import keys into a HD wallet... You can sweep them, but you can't import them. A non-HD wallet should solve this.
Same stands for electrum... If you restore from seed, you create a HD wallet which can only be used to sweep... If you create a wallet entering the private key, you have a non-HD wallet in which other private keys can be imported.

BTW: what's the format of your private key?