Answer from cs61c-es (Meng Chang Bruce Lin 16075852) for Question 1
MIPS instructions are represented by field types of I, J and R. These field types has different bit patterns that presented opcode, registers, shtamount, functions and immediates. Since they are presented in bit, it is possible for someone to alter the bits since one can just change 1 bit, then MIPS will interpret it very differently. Someone can try to access restricted memory address which will mess up the operating system and the memory management. That is when the computer crashes. 
One way to minimize the threat is to specify the opcodes for each instruction. We can also limit how a person can change the bit patterns. Or strictly following the rules of making instructions. The main trade off will be less elbow room for programmers or users to work with instructions that have too many restrictions.