You are not wrong, you are a right kitten.
But your hypothesis is indeed wrong.
Cf my post about protocols on the previous page to understand why.
The additional buttons are obtained only by following a protocol.
If pin 7 is set to 0v, the protocol is not respected and although I am not sure what the controller's reaction will be I am certain this won't make the additional buttons accessible.
Just invert pin 5 and 7, that should just work as normal. And when WHDLoad implements the protocol you will have access to all three or six buttons.
Erratum/Update: in the case of a regular 3 button pad
you are possibly right. It indeed uses a real multiplexer and does not care for the protocol. But it will need power to the controller for the multiplexer to work.
However you would have to make sure to set the pin to 0v instead of letting it floating unconnected since it is not guaranteed that an unconnected pin will be set to 0v.
This
might also work with 6 button controllers but only by pressing the mode button when powering the computer since this signifies the controller to behave like a three button one, however this is not guaranteed: you will have to try.