Boolean majority function
1) It shouldn't apply at all to a multiplexer if we're talking about the same thing.  A multiplexer is (to me) a device that has many inputs and one output and uses selection pins to tell it which input should be routed to the output.

2) If the two sets of gates implement the same boolean function then it does not matter what gates are used in the implementation of that function.  There are multiple correct implementations (but not multiple correct implementations that minimize the number of gates used).