MacBook not booting up, keyboard?

I have a white MacBook from the middle of 2009. This morning when I woke up and went to use it, I noticed that it was turned off. This is abnormal because I generally always leave my computer on and I would remember if I turned it off the night before because it's so out of the ordinary.

So I try turning it on and the computer gets to the Apple logo with a spinning wheel. Shortly after though, it shuts down before it even gets to my desktop. My computer is out of warranty so I decided that since Apple almost always charges an exorbitant amount of money to fix things that are out of warranty, I figure that I'll disassemble part of the laptop and see what's wrong.

Long story short, I disassembled it and took out the keyboard that came with it. I then proceeded to plug in a non functioning keyboard (friend spilled lemonade on it) taken from my friends old MacBook and voila! It worked. This is really weird though since my keyboard that was in otherwise perfect condition didn't work but my friend's keyboard which had been damaged by liquid resulting in some keys not working worked perfectly and allowed it to boot to the desktop. Why is the keyboard even affecting the MacBook's boot up?

So it looks like I'll have to buy a replacement keyboard for my Mac but I think first, I'll stop buy the Apple store to see whats up and if they can possibly replace it for free. This is just a really weird thing that happened considering that I didn't spill anything on it the night before and it randomly decided to stop working.

So here are my questions..

What possibly caused this to happen?
Can Apple tell if i disassembled the computer? As far as I could tell, there weren't any indicators showing that the device had been opened like the iPhone.
What is the likelihood of Apple deciding to fix it for me assuming they don't know that I opened it up?

I typed this all on my iPad so I think I may be forgetting some stuff, it's difficult to scroll up and down in this text box and proofread everything.