Being an AU has no influence on your chances of being approved for any credit card. Approval will depend on several factors, payment history, inquiries, util ratio, etc. As an AU, it will, however, make a positive mark on your length of credit history.
Regarding back timing with AMEX, the rules changed last year, I think? It would help the length of credit history if they did factor that in. Personal experience, I started with AMEX in 1996. Bad decisions and immaturity led to financial downfall. BK, Ch 13 and gave up all credit cards in 2005. Jumped back on the wagon in 2015 to avail the bonuses all the companies lure you in. Reinstituted relationship with AMEX in 2016. With the new rules, my length of credit history was ZERO for 10 yrs. They did, however, recognized my past with them. My cards says Cardmember Since 1996. And the bonuses earned will not go to you as an AU, they will go to the primary cardholder. I'm sure your mom will give them to you anyway. Including bonuses she earned herself, I'm bet. And once you apply with your own SSN, you will earn the welcome bonus offered.
Yes, I value and protect my financial relationship with AMEX. They do their own thing like you said. Primarily, like most other financial institutions, your ability to pay and present financial habits (new accts, inquiries - all I'm very guilty of), will determine if you get an approval or denial.
19 new accounts in 8 months - AMEX gave me 6, Chase gave me 4, Wells Fargo 2, Citi 1, others are rebuilders and secured cards, I don't know If there are SET standards they follow. My rule: keep util ratio 1-2% (currently 0%), stable income 110K, paid off all collections, disputed them and had them all removed from all 3 CRAs.
Good luck to you. Hope you will get your BCE and your foot in with AMEX.