Macbook Pro 15″ replacement Keyboard / Keys

Could it have "melted" due to the screen heat?

No. Impossible.

Also, is there a way to easily clean the keyboard?

You can pop out the keys if you like (see this YouTube clip). I would advise against pulling out the entire keyboard as you won't be able to clean it any quicker and may damage it (depending on your dexterity) in the process. It is also a lengthy process that requires removal of the logicboard (hit up iFixit for details). Technically, the keyboard is attached to the top panel of the notebook (unlike the pre-unibody MacBook Pros).

As for a cleaning solution, you can use any monitor cleaner or a high concentrate (99%) isopropyl alcohol (70% will if you can't find the 99%, but not preferred). Apply it using q-tips or a small micro-fiber rag.

Finally got it. I was pressing option keys lot of times and pressing it 5 times enables mouse keys (which disables keyboard keys).

To reverse, go to "Settings -> Accessibility -> Mouse and Trackpad" and uncheck "Press the option key five times to turn Mouse keys on of off" or simply uncheck "Enable Mouse Keys."