Answer to "How do I remove the keypad for its replacement?"Looks like your keyboard is held in by 5 plastic tabs( above the following keys: ESC, F5, F9, Prt Sc, and End). If you depress them with a spudger tool you should be able to lift up the keyboard and disconnect the ribbon cable underneath. Looks like this guide is fairly accurate Asus K53E-MT1 Motherboard Replacement (Step 7). I would just advise against using a flat head screwdriver, you'll want to use something plastic to prevent damage to the laptop.
Answer to "saving my data after installing the ylod repair kit"If you use the same hard drive in the same PS3, you should not lose any data. Also, the backwards compatibility on the 80gb is written into the software of the system which would also not be affected. Just make sure you remove the hard drive and protect it properly before you re-flow the solder and you should be good.