Answer from cs61c-cj (Peter Lau 16539384) for Question 3
The hazard unit inserts bubbles with a nop instruction or sll $0, $0, 0.  The hazard unit will insert a bubble only when it detects that it needs to insert one.  For example, if a register is read immediately after a lw instruction, then one bubble is used to delay the read instruction to a point in time in which forwarding can solve the problem.  It will not insert bubbles repeatedly because it will not have to.