If it works with the laptop's keyboard disconnected and a USB keyboard connect, the laptop's keyboard is likely messed up. The lemonade is closing the circuits on the keyboard making it appear that keys are pressed when it's turned on. This same thing happened to two laptops I've worked on recently, one with soda water spilt on it and the other with coffee. Surprisingly, I was able to get the coffee keyboard working again, but the soda water one seemed to be permanently damaged as nothing I did to it would get it working correctly again. It ended up having to have the keyboard replace at a cost of $50 if I remember right.
If you can get under the keys and clean the keyboard, there's a chance you might be able to make it work again. The keys come off most laptop keyboards, but the plastic bits are very delicate and can easily break if removed too forcefully.
The reason it boots if you press other keys when booting is because it basically ignores the signal from the stuck key when another key is pressed and released. If you can do this to get into Windows, open Notepad and test all the keys (or download some keyboard testing software) to determine which keys are the culprits and see if you can clean them.
