2017 13" MBA - cost and time to do keyboard replacement?

It depends on whether the repair is performed at the AASP's shop or whether the laptop is sent out to an Apple repair depot. If the AASP performs the repair themselves in their shop, then it can take as little as two days depending on AASP schedule and assuming the AASP receives the part in a day or two and nothing else is discovered or goes wrong. If the laptop is sent out to an Apple repair depot, then that usually takes about a week as least in the US. Repairs for other regions will vary.

If the laptop is sent to an Apple repair depot, then the price of accidental damage may be near or even more than your $400 limit (Apple has a tiered pricing structure for repairs sent to a repair depot). You will need to get an official quote from Apple or an AASP to be sure. There is always a possibility that the Apple repair depot may discover more extensive damage and requote the repair. If Apple requotes the repair and you decline, then you will still be responsible for about $120 in non-refundable shipping and labor costs. Most AASPs have a lower diagnostic bench fee if they perform repairs in their own shop, but are at the mercy of Apple if the laptop is sent to a repair depot.

If your Apple Store is difficult to deal with, then check out any local AASPs.

You should try to get your organization to set pre-approved repair cost limits to make your job easier. It usually makes sense to repair anything that costs less than half the price of a new computer, although there are exceptions when accidental damage is involved or if the computer is older and may not be used for much longer. It certainly makes your job and even your experience with Apple or an AASP much easier for both parties.

It sounds like this might be the result of a small liquid spill (or splash of liquid). You can try removing the key caps and scissor mechanism for those keys so you can clean up the liquid residue. A little bit of Isopropyl Alcohol on the scissor mechanism and key cap will easily remove the residue. You may need to use a Q-tip to clean the reside from the key bay on the Keyboard.

You just need to pay attention to how the scissor mechanism is put together (it only goes one way). You also have to be careful when prying off the key cap since if you pry on the wrong side you can break the tabs on the scissor mechanism. The Enter & Shift keys also have metal stabilizer bar that can be tricky to reattach without some practice. Check out iFixIt.com for a guide on how to remove the key cap for the MBAir.

As far as cost, it will depend on what Apple or an AASP charges for the part and for labor. It may also depend on whether the repair is performed at the store/shop or whether the laptop is sent to an Apple mail-in repair depot. If it is deemed Accidental Liquid Damage, then the cost could be more. An AASP repairing it in their own shop may charge as little as $250 for parts and labor to replace the Top Case/Keyboard Assembly (even if considered accidental liquid damage). An AASP may charge less to try to clean a couple key caps if that is all that is wrong with the laptop.