1) for most anonymity, is the default installation enough or are there any extra steps needed to be done to whonix?
Hm. Define most and needed. Going through pages warning, donot, pre install advice, post install advice, security guide, documentation and whatnot is certainly helpful. Probably I cannot phrase it any better than this...
This documentation is a crash course in anonymity and security on the Internet. Whonix is a technological means to anonymity, but staying anonymous is not just a technological problem: No tool is enough to keep you safe.
Anonymity is a complex problem without an easy solution, and security is only as strong as its weakest link—often the user. The more you know, the safer you can be. Fortunately, basic knowledge and caution will go far.
2) downloading whonix via tbb enough to hide the fact of using whonix?
3) is mac address changer included or needed within whonix? is it recommended for use within personal environment?
4) are there any known leaks/deanonymization of ip or host/vm machine or similar problems that is known or possible in latest whonix or are these only possible with an whonix bug?
At time of writing... No known leaks. Deanonymization requires indeed a bug in Whonix or any of its components and/or capability to break Tor.