Demonstrating mnemonics abilities if demanded to do so is easy and there are various outside mnemonics experts that can attest to the fact that it's possible to do so.

Yes, but you need to be demonstrating the forgetting exists and is accidental. 'Oh, I'm sorry judge, I totally forgot! also, this is totally not destruction of evidence so please don't have me up on either contempt of court or obstruction of justice!'

You could also take a polygraph to verify that you tell the truth about having deleted the password.

Polygraphs aren't very reliable for verifying you're telling the truth and I think judges know that by this point. Plus, that could easily backfire the other way: you could be nervous enough that your readings are consistent with lying.