Answer from cs61c-eu (Tian Ming Ouyang 16017341) for Question 1
While and do while is pretty much another sugar coded if, because if the the function that can be used to write the other two.  While and do while will be dropped since we don't need them all, in MIPS that is.  The designers made C like this because it's a higher level language than MIPS and they want people to read the code as close to human language as possible.  This way the code would be easier to read, plus easier to learn.