What are some good methods of recovering key function after a spillage?

Apple are very likely to repair this under Applecare with no charge. I have had several defective keyboards and stuck keys in the past. In all cases, Apple replaced the keyboards for free under Applecare.

Clearly your milage will vary, since if you have damaged the keyboard by misuse they are under no obligation to fix it, but in my experience, Genius bar engineers will always give you the benefit of the doubt.

There are LCI (Liquid Contact Indicators) on the topcase/keyboard of the MacBook Air which are quite sparsely placed. If these are tripped Apple will nearly always decline a repair, especially if the faulty module is the one with tripped LCI's. There's also a chance that there will be signs of liquid ingress on the underside of the keyboard.

Parts with tripped LCI's will not be covered by warranty until replaced.

However, there's a good chance that there won't be any sign of liquid damage. Plus Apple tend to be super nice to their AppleCare customers, so it's worth a punt.

The important thing is to deny all knowledge of a spillage

The best method is to take it to someone that knows how to repair keys on the mac. Repair shops generally have spares of the delicate scissors and keys (as they can break on removal) and they will need to remove the key cap to investigate whether the spill was related.

If the sensor contact that lies under the I key is shorted (while wet), you can expect a recovery, but that the liquid will be corroding things.

If the mac is totally dry and still not working, the electronics may need to be repaired.

Repair techs see this hundreds of times in a year - it's not so scary to them, so take it in and ask realistically how the repair would go. If things are corroded, going in to move parts might result in a non functioning machine - so it's a gamble based on exactly what liquid, where it went, how fast it dried and plain old fashioned luck.

Hopefully it's a piece of lint or something preventing the switch so learn how that key comes off and see for yourself or take it in if there are convenient repair options.