On Side B, it’s Side A that looks like it’s happening in reverse, but in actuality it’s you that’s inverted.
Not only does she not need the telling oxygen mask that would out her as another inverted version of herself, but she’d be able to speak normally.
As such, the inverted membrane of your lungs cannot handle standard air, and you’ll need to reverted to breath normally.
As such, the now reverted Kat 2.0 is now able to go to Vietnam, sans oxygen mask, and try to stop Sator from killing himself; while her original self goes on a trip with her son.