Although the AA rtw desk is extremely professional, there have been times when I have received differing rule interpretations from them.

To protect myself against the problems you outlined, I try to keep a print out of the current AA star file that presents the exact fare rules and make sure I can back-up anything "unusual" by referring the objecting party to the governing section. This goes a long way to smoothing out difficulties (particularly if I avoid taking a confrontational approach and/or avoid making the AA staff member appear to be stupid). Sometimes, I'll say something like, "yes, I was surprised that I could do this, too. It looks like it was a recent rule change (even if the relevant section has been included in the rule for years).