96w USB-C charger for a 2019 macbook air

Shouldn't be a problem. The MBA will "negotiate" with the power adapter and will only draw what it needs or can safely draw (USB-C cables can have current limits). The power adapter doesn't really control how much power is going into the computer. It's essentially just current limited voltage source (although USB Power Delivery complicates things), and the computer will draw as much as current as needs up to the current limit of the power adapter.

It shouldn't charge any faster though. The 2019 MBA is limited to 30W input.

Yes.