My question is how can we check the power of Wladimir? Is there some voting process like the election of US president?
AFAIK being the top D for bitcoin does not undergo any voting process. It simply is entrusted by the previous top D to a new person he sees fit to continue on working with the code. These guys are not regularly compensated and are rarely funded; they only rely to donations and whatnot so I don't think there's much power into being the top developer of bitcoin. You are simply known for your knowledge and ability to do changes on the code for its betterment, and even if you turn out to be a nefarious being, the code is open-source, and people can easily opt for a fork should there be any malicious part in the code that you have uploaded.
Wladimir maybe like a kind emperor, what if Wladimir gives the power to some bad guy who work with the miners to increase the block reward (and hence increase the total bitcoin supply)?
Remember today mining pool is highly concentrated and therefore controlled by very few people.
That wouldn't happen. Substantial changes in the code undergoes a voting process, or a consensus as it is officially called. Also, even though miners are hugely concentrated in China, they do not belong to a single entity, and you need more than 90% of the whole hashing power of the network in order to impose changes thoroughly. Even then, you would not be sure that everyone would follow your version of bitcoin. Heck, it took a good year before blockchain.info implemented bech32 searches in its own service...
Don't tell me something never happen, remember block reward is just a parameter in the code. In financial word, anything can happen without proper checks.
Unfortunately (lol), the code is regularly checked by thousands of people every single day. I'm not saying that it would never happen, but it really wouldn't.