Can the Mini 2's charging hub work off the Mavic Air 2's AC adapter?

Yes, that’s what I used to charge the batteries while using the one supplied with the fly more to charge the controller.

I just bought an extra 3rd party USB adapter in a shop. 5 V output, 2.5 A to charge the controller while charging the batteries.

I think the USB Outlet on the Brick is just a 5V outlet like most USB Outlets and I also charge the controller for the MM2 with the MA brick while I charge the MM2 batteries

If you mean the USB-outlet on the charging hub, this can be used as a power outlet. It will use the batteries to supply power to this outlet, so you can charge devices. The only thing is, while you are charging batteries, it doesn't supply power to simultaniously charge the controller. I tried that before buying an extra adapter.

Thanks everyone for your advice!