For what it is worth I agree with SB. I think you should stop focusing on the "chargeback issue", as relevant as it may seem, it is not all that relevant to the liability for the account from AMEX's perspective.
Yes, it was probably wrong, not nice, not justified, not reasonable and perhaps not even legal, but it has very little to do with who AMEX can chase for the balance of the account. You can choose to keep pursuing the chargeback issue. However, in my opinion you would be flogging a dead horse.
I would, instead, gather my evidence which supports the assertion that the purchases were made on behalf of, and as instructed by, other directors in the Company. This is a far more relevant aspect of the issue and depending on the evidence you have, would have a far higher chance of convincing AMEX to stop pursuing you for the account. Although, just my two cents, you can choose to continue with the chargeback issue if you wish.