I spilled coke on my keyboard, and instantly flipped the laptop upside down to avoid it going underneath the keyboard. Dried it with some paper towels, and resumed work with slightly sticky keys. A few days later, I was starting to get annoyed with some of the keys that were getting worse, not springing back up after being pressed, etc.. the left arrow key was not working anymore. So removed the keyboard and cleaned it with warm water and dried it thoroughly and re-installed it. It got even worse although the keys weren't sticky anymore, so I immediately disconnected it thinking it wasn't completely dry. The next day I retried and it didn't work either. My colleague at work is telling me that the interface on the motherboard must have been damaged. Is that possible? Can a wet keyboard damage the the circuitry on the motherboard? I'm absolutely sure no coke got inside, I disassembled the laptop and there were no traces coke inside, it was a very small spill directly on the keyboard.
It depends on the model of the laptop and the amount of liquid spilt. Most laptops protect the major components from this type of accident by designing the keyboard in such a way that prevents liquid getting anywhere near the internal components, but you will always end up with sticky keys when spilling coke because of the amount of sugar in it (when it dries it just turns into sticky goop, as I'm sure you are aware).
To be honest, seeing as the problem is limited to a few keys not working, it sounds more like the keyboard itself needs replacing. If the motherboard was damaged your laptop would no longer function. Laptop keyboard replacement is generally very straight forward and I wouldn't have thought it would be particularly expensive to get a professional to do it.
Check if you didn't loosen connections between keyboard and motherboard (on both ends). If it's loosen and doesn't have good contact then some/all keys could stop to work.
It could damage motherboard but it's unlikely:
probably it would differ in symptoms, some other things would stop to work more likely than connection with keyboard
it's much more chance to damage motherboard if you try to work on it when it's wet than if you turn it off and dry (as you did)