I would call and ask for them to reconsider based on changes to your CR. They will probably ask that you fax an updated CR. I'm not sure if it would be a hard pull. Others can probably chime in.
Don't give up, you've got your foot in the door with AMEX and that's a good thing.
I'd call back and ask them to pull a more recent CR. You have nothing to lose.
I would just give it a month or two and the report will get updated. I have not really heard of Amex pulling a report on the spot for someone, it's like recon and application it goes to a back office (underwriting that usually you can't talk to) you recon for it to go to that back office and they might realize what's been happening on your reports a little bit better and cancel the account.
How long have you had this account open?
I'm surprised when you dipped into the 500's Amex didn't take your CL down to $500 if not close.
Why would Delta and Amex not allow an account to have CLD or closed if the credit got bad? I have heard of not PC Amex card but nothing MAKING a bank keep an account open?