Answer from cs61c-bg (Spencer Ahrens 16187781) for Question 3
The add unit is hardwired to interpret the address for a branch jump by adding the branch immediate and current PC.  They can't be combined effectively because the adder has very specific hardwired inputs that it is working with, but the ALU is more flexible and can do different relations.  If they were combined it would also slow down the system because they would have to run in series rather than in parallel.