Can you give examples of how you resolve conversations with designers when you seriously disagree with a changelist? You guys sound like you'd run into a lot of issues.
Typically, the designer (and their design lead) has the final call. Designer and GAT member frequently talk through the changelist, the pros and cons, design intent and expected result. If there's a disagreement on these, then we can do any number of things including : bring in more opinions from more senior members of GAT side or design side (including going all the way up to meddler if it's important enough), put something in test to validate the hypothesis (with test goals), ship it with contingency plans (eg. Make a winrate shotcall. If this is more winrate than predicted, then revert it or pull back). There aren't too many issues because everyone has good interpersonal skills and it's handled in a professional manner.