Answer from cs61c-cf (Aspandiar Dahmubed 16812056) for Question 1
To avoid saving and restoring a register whose value is never used, which might happens with a temporary register, MIPS offers:
10 temporary registers ($t0-$t9) that are not preserved by the called and 8 saved registers ($s0-$s7) that must be preserved on a procedure call. 
This reduces register spilling.