Sorry, this won't help, but I'm anxious to hear the resolution as I have a very similar problem with a 74HC4067 16 channel Mux/Dmux.
I also share the SIG pin between a analog pin to get input and a digital pin for output. When getting input, the digital pin is set to OUTPUT which should make it Hi-Z so it doesn't interfere with the analog pin. But it does interfere. (Sounds like your interference is on the input cycle though.)
I've tried turning off the internal pullup on the digital pin (setting it LOW while set as OUTPUT) but no joy.
(Hope this doesn't interfere with your answer.)
[edit]BTW - you might want to try a different analog than 5 - maybe I2C is an issue. [/edit]