I didn't save the key anywhere. The string of random characters that make up PrivateKey is created using the BrainWallet method, the salt of which is the password I use, consisting of 20 characters (including special characters and numbers), so there is no chance to guess. This was done manually and offline using a tool of my own authorship. I know the mechanisms of private keys very well
So you didn't actually create your private key with Electrum? I'm just thinking out loud here,
but it's kind of pointless to generate your private key offline only to later import it into a hot wallet.
I think the hole is in this scenario. I guess OP's PC was compromised with key logger. I also think that it is pointless to generate a private key then later to import it to hotwallet. So I guess that import stuff is the source of all those problem.