1.Is any IR protocol based remote buttons have unique IR hex code, eg LG TV power button gives me hex code 20DF10EF and it was also same as in LIRC database, is it unique among other TV brand power button?
It should be, but not guaranteed.
2.Is all LG brand TV controlled via any LG TV remote? means i will decode one remote of any LG TV and expecting, it will work on most of LG TV.
How to extract code from LIRC?
You need to study it...
Search for "sb projects IR"....its agreat site that will explain a lot about the make up of IR signals. You will be able to answer your questions above, after doing some research there.