I believe you want one of the newer Arduino or compatible controllers that can generate USB mouse and keyboard events. I've not used the capability, but the Leonardo, and Due support it as official Arduino boards. In terms of compatibles, the PJRC Teensy 2.0/2.0++/3.0/3.1, the Adafruit Flora/Trinket/Gemma, and the Digistump Digispark/DigiX also support the USB keyboard and mouse. The Arduino Uno does not support it without using a USB host shield. I don't recall if the Mega supports it.
You can get breadboard adapters for 3.5mm connections to hook up the switches. For example, this board from Sparkfun: https://www.sparkfun.com/products/11570
. You can also get the ends you solder wires to for a panel mount 3.5mm at places like Radio Shack (if you live in the USA).
One thing that might be an option is touch sensing. The Teensy 3.0/3.1 have certain pins that can do touch sensing and return an analog value based on how close your fingers are to a wire connected to the appropriate touch pin. You don't have to make contact at all (obviously, it will take some time to figure out how close to get and what values returned mean what). Unfortunately there isn't a whole lot of documentation for it. There is this thread for instance: http://forum.pjrc.com/threads/13987-Teensy-3-0-capactive-touch
. I don't think any of the other Arduino or compatibles can do this directly. There are ways to do it on the other platforms, using two pins.
I recall that there was a post within the last month or two, of somebody who was making an interface for his disabled friend in this forum. You might try searching for it. I tried some simpled minded searches, but I wasn't able to find it. If you find it, you might see some ideas to try.