Answer from cs61c-an (Ben Weil 16819970) for Question 3
The Add unit is used to calculate the target of a branch.  It would be possible to use a single ALU to do the addition of the branch, but the Add unit is also used for other memory address purposes, such as incrementing the PC by 4 bytes to the next piece of information in memory.  However, using the same unit would greatly slow the time of these operations, such as PC+4, which occur extremely often.