Answer from cs61c-ew (Joo-Rak Son 16103505) for Question 3
To insert bubbles, the hazard unit forces stages of the following pipelined instructions to repeat. Such repeated work is what a stall looks like, but it simply delays the following pipelined instrcutions from advancing forward.
What prevents the hazard unit from inserting the same bubble over and over and over is that there is no need for such thing. The most traumatic hazard generators like branching and reading after loading only requires only one bubble to be inserted in following instructions.