Answer from cs61c-cf (Aspandiar Dahmubed 16812056) for Question 3
The add unit is used to determine the branch offset that needs to be added to the PC + 4 counter. Yes, it would be possiblye to use a single ALU, however, that would use extra and more comlicated hardware and would not favor simplicity. Furthermore, since we're interested with the branch offset anyways, it would be ideal to add it to the PC counter and make it a separate unit.