Answer from cs61c-ba (Zhou Zhou 16250641) for Question 2
$t0 will still need to acquire a value from the $s1 register before it can be used to be added in the following pipeline.  In this case a stall is required before excuting the second instruction make sure that $t0 had taken on its new value.  Or the compiler can also swap the second instruction with another instruction at a later place to allow time for the first instruction to fully take place.