My question; who decides and re-decides for Bitcoin?
Since Bitcoin is decentralized the answer is: The entire bitcoin community which consists of miners, nodes and the economic majority.
1. Did Satoshi make all the decisions perfectly as relating Bitcoin before his disappearance?
Yes and no. The overall design you see today that we call "Bitcoin" is what Satoshi created and it has been working very well. There are certain smaller details that needed to change to make Bitcoin stronger and Satoshi's decision weren't very good.
2. If Twitter, Facebook, Amazon, Tesla to mention but a few, including other cryptocurrencies are embattled, we all know the faces that will spring up to defend the project. What if the world unites against Bitcion and demanding a representative to reach a consensus, who will represent Bitcoin.
Can anyone force the torrent network to have a "representative"?
That's the nature of decentralized peer to peer networks, they don't have any.
3. Is Bitcoin a legal entity? If yes, who represents her in the eyes of the law?
The Bitcoin owner and maybe not the Bitcoin itself. It therefore means, that the Bitcoin owner can commit a crime with Bitcoin and not Bitcoin committing crime itself? Please, can someone address this for me?
Bitcoin is a currency, or a tool and like any other tool it can be used for both legal and illegal actions. When used for anything legal, it is fine and when used for anything illegal the person will be punished when caught by the law enforcement.
4. Dear Bitcointalk users, are there some decisions you feel Satoshi should reappear and address, or are all things pertaining BTC in order?
No, because as I said above bitcoin is decentralized and Satoshi doesn't own the protocol to need to come back and dictate anything.
6. I read the controversy behind the creation of Bitcoin cash. If such scenario happens again, and more than 51% is on the negative side of the election, what will be the fate of Bitcoin?
The bcash scenario was repeated about hundred times in the past couple of years. Scammers will always copy bitcoin and abuse its name to steal other people's money.
As for your second question, I don't think we will ever see a split in Bitcoin that big. The community will always go with what majority wants or not change at all. Take SegWit2x fork from 2017, it had almost complete support from all miners but not from the rest so it was abandoned.
7. I wouldn't ask how will the decision be made when 21million bitcoin is mined, because I know it can be decided due to open source code of bitcoin. What if before 100yrs time, a new world order emerges that will render Bitcoin a menace, who will order the halt to save humanity?
We can't even predict what would happen in 100 seconds from now, how do you expect predicting 100 years?!!