As the Mavic Air has a standard USB micro port for charging the controller, does that mean it can be done from any USB port? I have one of those multi USB charging hubs that supplies 2.5a, can I use that safely?

Yes I think, the two USB port on the Mavic charger are "regular", they can charge my phone, so I guess my phone charger can charge the remote too. You can even use a powerbank

No problem, you can use other USB adapters.

i think usb c  can do up to 5 amps so test, the charger that comes with the drone might be better than 2.5 amps
i use my phone to test, it will say slow charging for 1.5 amps or so, charging for 2.5 and fast charging for higher than 2.5