Since you were talking about legacy *A carriers and needing to check in at the US ticket counter it was less than clear - none of that involves one world. Any of the few remaining US partners - *A or merely a partner - should have the same ability to credit to US if you use your US "one world" FF number (the one that's the same as your "*A" FF number) that they've had since before US left *A. As long as a carrier, *A or not, has a relationship with US nothing has needed changing with that carrier's systems. So checking in at their ticket counter should be fine, just as it was before US changed alliances.
Jim