1. Does miners get rewarded for their effort (computing power) that have been contributed during mining even if they never get to mine a block?
As pooya87 mention, there are 2 ways of mining:
1.- Solo mining. Is where you are running your mining farm and each block you fin it's yours.
2.- Pool mining. If your mining power is low then you can join to a pool where the mining reward gets shared between all the pool participants
If you are using any of those 2 methods and you or your pool doesn't find a block, then you don't get reward.
2. If the owner of the 51% of the total computational power decides to cheat, can he/she change the value of the bitcoin reward (say from 12.5 to 1000 bitcoins) and add them under his account?
There is an attack known as the 51% attack, but it doesn't consist in changing the mining reward, it's about change the blocks mined, that way you can make an attack that would looks like a time traveling where the attacker modify the old blocks with a malicious intention.
