No problem Rostos, and please don't apologise. I understand this must be incredibly frustrating, particularly since you have followed the rules and it has been thrown back at you for no good reason.
If you have made a complaint with FOS in relation to AMEX contacting you about "the debt", I cannot imagine FOS instructing AMEX to cease with their recovery action. At the end of the day, through no fault of you (or AMEX it would seem) AMEX is out of pocket and an account which you are partially liable for remains unpaid, despite it being paid previously.
It is probably worth noting again that AMEX would not have had a choice or any input in relation to the chargeback, it is between the Director and the Company's bank, both you and AMEX are powerless in this regard. It is probably best to view AMEX as another innocent party to this situation, with the main "culprit" being the Comany/Director A/B. Since AMEX is an "innocent party" I think FOS will not be of much use to you, rather you may have more luck reporting the Bank that processed the chargeback to FOS or APRA, although this would not be my chosen method to resolve the situation.
It is good to hear you had a lawyer contact AMEX, although I think it is quite clear that if you were named on the account then it is quite straightforward why they would chase you for an unpaid account. I agree the Company is also liable, and should be held solely liable for the debt.
Anyway, in order to resolve this I would -
1. Contact the Directors if you are still on speaking terms with them and see what the issue is, alternatively send an email so there is a paper trail.
2. Send AMEX a letter outlining your position, why you're not liable and include a cease and desist letter in relation to the recovery action. Don't forget AMEX likely (given them seem quite useless) are probably not aware you are no longer a director of the business, once this is made clear to them in writing it may see them stop with the recovery action.
3. Failing that, and if AMEX continue to pursue the debt I would bring an action against the Company in the local magistrates Court to recover the balance of the account. Depending if the amount was less than $10000 and if I were not a lawyer I would go to the Court unrepresented since it is quite straightforward if you bring it as a minor claim.
You are also not liable for the other parties cost if you bring it as a minor claim. I am unsure on the exact rules for each state however if you tell me what state you're in I will gladly explain the process. If it is more than $10,000 consider applying to legal aid for assistance with a civil dispute, this is usually means tested and depending on your income they may agree to represent you for free. If not, you would need to contact a lawyer.
At the hearing in the Magistrates Court you could present the evidence you have that the Company agreed to pay the AMEX account (which you said you have) and the Court will hopefully make an order for the Company to pay you the amount of the AMEX account which you can then use to pay AMEX and end this fiasco.
Hope that helps