Answer from cs61c-el (David Tung 16479725) for Question 1
No, they cannot.  Every MIPS instruction must have its own unique bit pattern for the sole reason that if they did not, then the computer would become confused as to which instruction to execute.  This is accomplished because MIPS instructions are expressed as machine code in a total of three different formats, that all have different "flags" to differentiate them from others.  For example, an R-format instruction always has 0 as the value for its op field, which differentiates it from the I-format and J-format instructions.  To differentiate between R-format instructions, they have additional unique values for their funct field.  This similar convention is repeated with the I-format and J-format instructions.