Can you replace individual keys on the macbook air?

Possibly. It depends on the exact model of the laptop as well as what is broken.

If the scissor mechanism or butterfly mechanism is not damaged, then it is possible to replace the key cap. However, there may be more than one type of key cap for a particular model Mac so you would need to make sure to get the proper key cap to fit your specific keyboard's scissor/butterfly mechanism. Some key caps can be difficult to install such as the longer keys (Tab, Shift, Delete, Command, Spacebar, etc.). Also, it is very easy to damage a key cap when removing them and even when reattaching a key cap (one side hooks/slides in while the other snaps into place). Some laptops may also have different fonts, emblems, and even variations in the black coloring of the key cap which can stand out when paired with the wrong keyboard.

If the scissor mechanism is damaged, then it is also possible to replace the scissor mechanism, but you need to make sure to get the correct one for your specific laptop since Apple typically has at least two different & incompatible styles of scissor mechanisms & key caps. However, you want to make sure the metal brackets holding the scissor mechanism are not damaged nor the silicon cup.

If a butterfly mechanism is damaged, then you will need to have the whole Top Case/Keyboard Assembly replaced. If the key switch is bad on a butterfly keyboard, then again the whole Top Case/Keyboard Assembly needs to be replaced.

If the silicon cup is damaged or missing on keyboards with the scissor mechanism, then sometimes it is possible to attach another silicon cup, but this is difficult to do and needs to be done properly or the silicon cup will fall off or you may end up damaging the keyboard which requires replacing the whole Top Case/Keyboard Assembly.

In theory an Apple Authorized Service Provider should be able to replace individual key caps (scissor or butterfly keyboards) and even the scissor mechanisms, but it is not cheap since Apple only sells the a complete set of key caps for the entire keyboard. Judging by posts on the Apple forums it seems Apple and AASPs don't seem interested in replacing just key caps.

Yes, but depending on what year MacBook Air, you might want to take it to a Apple Store. Are they broken?