Answer from cs61c-ck (Allen Lee 16583554) for Question 1
One convention is that the registers $s0-$s7 are preserved after a function/procedure call, and $t0-$t9 may not be preserved after a procedure call.  This convention reduces register spilling because when we invoke a procedure we need only store the $si registers to memory and load these back when the procedure returns.  In the example in the book, it stored and restored two $ti registers, which is not necessary according to the convention.