Answer from cs61c-ew (Joo-Rak Son 16103505) for Question 1
No. Though there are three different types of MIPS instructions, and even if their contents may be the same, the "op" field of the instructions of different types are distinct so the computer knows how to take care of the rest of the fields. For the instructions of the same type, there is no chance of ambiguity either, since unique combination of "op" and "funct" and other fields correspond to the specific instruction.