Congratulations … your pretty close!! And you are the first person, as far as i know, that is using a generic MCP23017 module from ebay. I supposse something like that:
Very good idea!! 
Yes. My experience with AudioInjector soundcards has been a pain too … although when finally you get it working, the sound is not so bad … and audio input is something nice to have 
Yo could use one of them for a “panic button” … as explained here:
Regards,