While I believe Rob's answer has answered your question, I thought it might be worthwhile to comment and perhaps a different explanation, albeit with the same view, might be of some assistance to you.
Firstly, you mentioned that Director A and Director B were the major shareholders of the Company and all transactions were made as directed by Director a and Director B - this is irrelevant and would not have any impact on your liability as a Director. Perhaps better explained as follows: If the Company engaged in unconscionable/illegal conduct that seen all three Directors personally liable, Director A and Director B would not be any more or less liable than you because of their position as majority shareholders.
You may have sought approval from Director A and/or Director B for the transactions, but you could have used the card without said "approval". Furthermore, you stated the AMEX statement had the name of the Company, in addition to your name on the statement. I assume then, that the other two directors were not named on the statement. This begs the question why you needed approval to use a card which only your name and the name of the Company was associated.
I agree that given the nature of the dispute cannot be established, all of these comments are guesswork. However, for what it is worth, in relation to your "building example", the builder would need to sue the customer and then pay his AMEX bill.
The customer would never be liable for the AMEX bill, only the cost of the work done. It is therefore possible that the Customer could call his/her bank and request a chargeback based on the fact he never authorised the transaction. This would not be fraudulent AMEX would then chase the builder who would chase the customer.
In reference to your comment about raising disputes "left, right and center" yes, you could, however the bank would likely close your account if the disputes were unfounded or fraudulent. In fact, ANZ helpfully outline instances where they will process a chargeback on their website -
'Chargeback' is the term used for debiting a merchant’s bank account with the amount of a transaction that had previously been credited. There are a number of different reasons why a transaction will be charged back, but they mainly fall into two categories:
- Where the merchant has made an error at the point of sale, for example, an expired card has been used;
- The cardholder or the Card Issuing Company (“the Issuer”) is disputing the transaction. For example the card or cardholder were not present at the point of sale and possible fraud may have taken place.
In this instance the "merchant" is AMEX and the Card Issuing Company is the bank which paid AMEX the funds. FYI it does not need to be a "card" transaction, the same applies to BPay and normal direct deposits.
Hope that helps